Gas Analysis and Diagnostics

At a state-of-the-art diagnostic site, a gas analyzer is one of the basic instruments.

Unfortunately, many automotive repair specialists still associate the gas analyzer with carburettor adjustment. This is not the case.

Certainly, exhaust gas (EG) emission control is a critical feature of the automotive gas analyzer, but it is not the only one.

The device is able to solve a wide range of tasks to study the condition of the engine and its systems. It is an abundant source of diagnostic information. It may safely be said that the gas analyzer is one of the diagnostician’s basic tools.

As a doctor needs a patient’s analyses to make a diagnosis, a diagnostician needs “analysis” data to detect the engine’s “diseases”, because the EG composition directly depends on the engine’s condition.


Of all the components of exhaust gases, the first models of gas analyzers used for engine adjustment measured only the CO (carbon monoxide) concentration. These were single-component instruments.

Analysing the CO concentration allowed the qualitative ratio of the fuel-air mixture to be assessed and was mainly applied in carburettor adjustment. These gas analyzers had a needle display and worked by measuring the electric conductance of a platinum coil in a carbon monoxide medium.

By the 1970s, the need to control toxic automotive emissions became urgent. The state of the art in those years allowed the creation of two-component automotive gas analyzers, which are capable of measuring another harmful component – unburnt fuel designated as CH. Those devices worked using infrared spectrometry of the gases in question. This technique has continued to be used to this day.

Further development of automotive gas analyzers led to three-, four- and even five-component instruments that can measure not only concentrations of the abovementioned carbon monoxide CO and hydrocarbons CH, but also carbon dioxide CO2, oxygen O2 and nitrogen oxides NOx, as well as calculate the air-fuel ratio in the initial fuel-air mixture.


The gas analyzer’s spectrometry unit works based on the effect of partial absorption of the energy of the light passing through the gas.

The molecules of each gas are a vibrating system that can absorb infrared radiation within a strictly specific wave region. If we let a stable infrared flux pass through a flask with gas, a part of it will be absorbed by the gas. Moreover, only a particular portion of the light, called the absorption peak of the gas, will be absorbed. The higher the gas concentration in the flask, the larger the absorption observed.

The fact that various gases have different absorption peaks lets us measure the gas concentration in the mixture by measuring absorption at the corresponding wavelength. In other words, we can determine the concentration of each gas in the EG by analyzing the decrease in the intensity of the light within the portion of the spectrum that corresponds to the absorption peak of that gas.

The device’s spectrometry unit is designed as follows.

Pre-filtered exhaust gases are pumped through a measuring cell, which is a tube with ends closed by optical glass. There is an emitter on one side of the tube. It is a coil heated by an electrical current with the coil’s temperature being strictly stabilized. The emitter generates stable infrared radiation.

On the opposite end of the tube there are light filters installed to separate the required wavelengths, which correspond to the absorption peaks of gases in question, from the entire light spectrum.

After passing though the filters, the light enters an infrared detector. The detector measures the intensity of the light and produces information on gas concentrations in the mixture.

This is how the CO, CH and CO2 concentrations are determined. Then the gas mixture goes successively from the measuring cell to electrochemical-type sensors, which generate an electrical signal whose voltage is proportional to the concentrations of oxygen O2 and nitrogen oxides NOx.

In modern instruments, CO, CH and CO2 concentrations are measured using this spectrometric method, while concentrations of oxygen O2 and nitrogen oxides NOx are measured with electrochemical sensors.

A microprocessor-based electronic circuit processed the signals from the sensors and the spectrometry unit in the modern-day gas analyzer.

Information on CO, CO2 and O2 contents is displayed on the device screen as percentages, while CH and NOx concentrations are represented in ppm (parts per million). This unit of measure is used due to the extremely low concentration of these components in the EG and the fact that it is inconvenient to use percentages for this quantity. Percentages relate to ppm as follows:

10,000 ppm = 1%

Thus, the amount of CH, for instance, in EG from a typical engine would be about 0.001%-0.01%. It’s difficult to talk about numbers like that in this field. That’s why it is common to use ppm.

The gas analyzer is a complicated device, with its quality determined by the accuracy and reliability of its components, especially the spectrometry unit.

In terms of design and technology, the spectrometry unit is so complex that only a few companies around the world have mastered the ability to manufacture it with acceptable quality.

Accordingly, gas analyser manufacturers use prefabricated spectrometry units, integrating them into their own devices. This approach has proven valuable; and you would most likely find a spectrometry unit made in Japan or the USA inside a device made in Russia, Italy, or Korea.

The spectrometry unit is expensive, comprising a good portion of the instrument’s price.

It is crucial to protect its long service life during operation. When they settle on the walls of the unit, particles, soot and moisture lead to significant drift in readings and even total failure.

Therefore, before exhaust gases enter the measuring unit, they undergo preparation that is carried out in several stages:

  • Rough purification of exhaust gas. This is performed by a filter installed at the device’s inlet or in a sampling probe handle. Large particles and soot are filtered out.
  • Moisture separator. There are a variety of designs. The purpose of this step is to remove drops of moisture that condense on the internal surfaces of the probe and the connecting hose, from the stream of gases. Removal can be performed automatically or manually by an operator periodically draining the condensate from a collector.
  • Fine filter. This is a final filtration to remove the smallest particles. Several filters may installed in series, one after another.


The specific instrument design affects its operation and maintenance recommendations. Operating an automotive gas analyzer is generally not very difficult and is performed by one operator.

Before taking measurements, the instrument’s zero position must be calibrated. This is done by pressing the appropriate button of the front panel. Some gas analyzers self-calibrate automatically at a regular preset interval. In this case, no operator intervention is necessary.

To take readings, the probe must be installed in the car exhaust pipe at a depth of at least 300 mm and fixed with a clamp. This large depth is required to prevent atmospheric air from flowing into the probe and creating unreliable readings.

Next, start measuring and wait for stable readings on the instrument’s display. It usually takes about 15 to 45 seconds for readings to stabilize, depending on the hose length and pneumatic system design, which can vary considerably in instruments made by different manufacturers.

Based on many years of experience operating gas analyzers, the following recommendation can be given.

After each measurement, the hose with the probe should be disconnected from the instrument and blown off backward with compressed air in order to remove condensate. You can usually see quite a lot of condensation cleared in this way. Certainly, the built-in moisture separator performs its function, but following this recommendation will increase the probability of fault-free operation.

Gas analyzer maintenance is mostly limited to periodic replacement of the fine and coarse filters. Recommendations on replacing them are provided in the operating manual of a specific instrument.

It is crucial to pay attention to the following: fine filters applied in gas analyzers are different from fuel filters; the latter cannot be used in gas analyzers.

Also, it is important to make sure that the filters are dry. Wet filters must be either be dried by air supplied in the direction against the arrow on the filter housing or they must be replaced.


The fundamental thesis to be announced before describing the analysis method for the exhaust gas composition is as follows.

Reasonable and correct analysis requires absolute understanding from where one or another component appears in the EG composition.

You need to have a clear idea of processes in cylinders and the engine exhaust system; while accompanying chemical conversions are based on this understanding.

At such an approach, the diagnostician begins to think and properly analyze the EG composition to see cause-and-effect relations. An approach like “if the EG composition is such, the present defect is such” is not practical and will not be regarded.

First of all, remember the composition of atmospheric air from your school course in chemistry. This is required to correctly understand the processes that take place in the cylinders and engine exhaust system.




Carbon dioxide (CO2)___________________0.03%

Other gases, mainly inert gases, exist in small quantities and are irrelevant for our purposes, as is argon. Numbers that are very close to those above can be seen on a gas analyzer’s display if you take a reading of “fresh air”.

Recall that an air-fuel mixture is burning in the engine’s cylinders. The reaction of fuel hydrocarbon oxidation follows the scheme:

CH + O2 => CO2 + H2O.

Let me remind you that it is common to evaluate the mixture composition with an air–fuel equivalence ratio λ. It is a ratio of the actual amount of air amount that enters the cylinders to the theoretical amount required for complete fuel combustion. Mixtures where the air amount is equal to that theoretically required are called stoichiometric mixtures. In this case, λ=1. If the amount of air exceeds the required amount, then the mixture is called lean and the ratio is in the range λ=1.0...1.3. A leaner mixture will not ignite. If there is less air than necessary, the mixture is called rich. Such a mixture features value of λ=0.8...1.0.

It would seem that when a stoichiometric mixture combusts, the exhaust gases consist of carbon dioxide CO2, water vapour H2O, and nitrogen N2. But it is different in practice. Under the high

temperature in the engine’s cylinder, the nitrogen and oxygen react with each other to create nitrogen oxides. These oxides are collectively designated as NOx and are measured by five-component gas analyzers. NOx creation is increases rapidly with the gas temperature and the oxygen concentration. The basic component in the nitrogen oxide mixture is nitrogen monoxide NO. After leaving the engine’s cylinders, it oxidizes in the atmosphere to become nitrogen dioxide NO2, which is much more toxic and combines to water vapour in the atmosphere to form acid rain.

Moreover, exhaust gases always contain hydrocarbons CH. These are original or disintegrated fuel molecules that did not combust, as well as the products of motor oil decomposition. Hydrocarbons appear in EG as a result of the flame extinguishing next to the combustion chamber’s relatively cold walls, in confined spaces like the space between the piston and the cylinder above the top compression ring.

Some CH are discharged due to fuel vapour being absorbed by the film of oil on cylinder walls during the fuel-air mixture intake and compression strokes. During the power and exhaust strokes, fuel vapours are emitted from the film. A similar effect involving fuel vapour being absorbed can also be observed in soot covering the combustion chamber’s walls.

Next, EG surely contain a product of incomplete fuel combustion – carbon monoxide CO. It is mainly created during combustion with insufficient oxygen, therefore, CO creation in petrol engines is mostly affected by the mixture composition: the richer the mixture, the higher the CO concentration.

Note that this component, I dare say, is the most dangerous in terms of human exposure. Carbon monoxide is colourless and odourless, but when inhaled it binds with blood haemoglobin and, at a high concentration, can be fatal.

Of course, EG inevitably also contain any oxygen that avoided the reaction. Note that there may be oxygen in EG not from the engine’s cylinders, but from atmospheric air coming in through leak points in the exhaust system.


Numerous studies have shown that improving the combustion process and optimizing the mixture composition and ignition advance angle do not decrease EG (exhaust gas) toxicity to a level that ensures compliance with Euro II standards, not to mention more stringent requirements.

In order to solve this problem, some have suggested treating EG further in the engine exhaust system. The devices to perform this additional treatment are called catalytic converters.

The basic parts of a catalytic converter are:

  • Housing made of heat-resistant stainless steel
  • Substrate/support, which is a honeycomb structure made of ceramics or corrugated foil with a thickness 0.1...0.5 mm
  • A washcoat with a porous aluminium oxide structure
  • The active catalytic layer

The substrate consists of several thousands of fine channels through which exhaust gases flow. The channels of the ceramic or metallic catalyst support are covered with a very porous washcoat, resulting in an approximately 7,000-fold increase in the useful area of the catalytic converter, which ensures the required mass transfer between EG and the active catalyst. The active catalytic layer is applied on the washcoat.

A three-component catalytic converter has an active catalyst layer made of platinum (Pt), rhodium (Rd) and palladium (Pd). The name three-component catalytic converter indicates that three chemical reactions are proceeding simultaneously and in parallel within a single housing.

For these reactions to proceed normally, a high temperature within 400…800°C must be maintained in the converter. At lower temperatures, the converter’s efficiency is quite low, while at a temperature over 1000°C would result in thermal damage to the active layer and even sintering of the substrate honeycomb.

Without going into the details of the chemical reactions that take place on the surface of the active layer, we can provide the simplified final results:

  • NOx are reduced to pure nitrogen N2 with emission of free oxygen O2
  • CO is oxidized to CO2, with consumption of oxygen O2
  • hydrocarbons CH are oxidized to CO2 and H2O, also with consumption of oxygen O2

A distinctive feature of the three-component catalytic converter is that its full-scale operation requires the engine to run on the stoichiometric fuel-air mixture. This can be explained by the following. Only at λ = 1 can we obtain the EG composition that contains the amount of free oxygen emitted during nitrogen monoxide reduction sufficient for full oxidation of CO and CH to CO2 and H2O, respectively.

This fact is so essential that it should be repeated: proper functioning of the catalytic converter is only possible if the engine runs on the stoichiometric mixture.

The literature even uses the term “catalyzing window”, meaning a range of values λ at which the converter is able to perform its function. Strictly speaking, this range is shifted from the stoichiometric toward the rich mixture and lies approximately within a range λ = 0.98...0.99. The engine control system is responsible for maintaining the mixture composition within the set range. This is why it includes a sensor to measure the oxygen concentration in EG.

We should also mention engines with direct fuel injection. Such engines in some modes can run on ultra-lean mixtures, resulting in a significant increased percentage of NOx. Therefore, to neutralize NOx, another catalyst, called an accumulating catalyst, is installed in the exhaust system.

For a deeper understanding of the catalytic converter, the following experiment was carried out.

They took a VAZ 2112 car equipped with the ECU VS5.1 with firmware V5D07X09 that supports fuel supply adjustment using diagnostic equipment.

It is equipped with a converter. CO, CO2, O2, CH and λ readings were recorded when adjustment factor varied between −0.250 and +0.250.

A pipe insert was installed instead of the converter; measurements were made once more.

The results are shown on the graphs. The solid line corresponds to a measurement with the converter while the dashed line represents measurements without the converter.

Graphs were plotted manually, with slight interpolation. One detail should be noted – for some reason the instrument showed the incorrect CO2 value when measurements were taken with the converter. This was likely due to the engine running at a low speed for a long time and thus decreased converter temperatures. With this in mind, we can look at the results and analyze them:

The first thing you notice is that values of λ are almost equal in both cases.

Within the rich mixture range, points formed a single line, while within the lean mixture range we can see variation on the measurement error level. The difference can be noticed only on the leanest mixtures, but it is likely that correct calculation of λ is just not possible within that range.

We conclude that regardless of whether you have a converter, the calculated value of λ remains the same. In fact, it could not have been otherwise, because λ only characterizes the operation of the engine operation, without regard to the presence of a converter.

The behaviour of the CH value is very curious. We can observe a classical dependence without a converter. But with a converter, the picture is more interesting. There has a strong effect within the lean mixture range. The characteristic valley that corresponds to the catalyzing window can be seen near the region of stoichiometry. A small increase in mixture richness relative to the stoichiometric mixture results in a dramatic jump of the CH value, which then becomes almost equal to the value obtained without a converter.

The oxygen content graphs are very similar. Naturally, when the converter is running oxygen is consumed, and this can be seen in the comparison.

The same can be said concerning the CO graphs. You can clearly see a range in the stoichiometry region where the converter’s performance is maximum and the graphs differ the most.

The CO2 graphs are also educational. The amount of CO2 in EG is larger with the converter. This can be explained by the fact that the converter converts the hydrocarbons and carbon monoxide contained in EG into CO2. Upon moving away from the stoichiometric range, both toward leaner and richer mixtures, the amount of CO2 decreases.

This is a critical issue: the maximum amount of CO2 in EG approximately corresponds to the stoichiometric mixture.


The air–fuel equivalence ratio λ deserves its own discussion. It should be clearly understood that a value of λ displayed on the instrument’s screen is a calculated ratio rather than an actual one. It is calculated by the gas analyzer’ processor based on the amounts of various components in the EG composition. This calculation is performed using the so called Bretschneider's formula:

This formula is provided for reference and will not be investigated here in detail.

The calculated value of λ will match the actual value only if the engine exhaust system is fully air-tight and the gas analyzer’s measuring elements are calibrated. If the exhaust system is not tight (there are atmospheric air inflows), the calculated value of λ may appear to be incorrect or even exceed all reasonable limits. This can be explained by the fact that the Bretschneider's formula uses the oxygen content in EG, and any excess oxygen would introduce significant error in the calculation of this ratio.


Considering everything above, we should state the exhaust gas composition of a fault-free engine. It should be said that hereafter we are going to speak about using a four-component instrument, since the five-component models, which show the NOx amount in addition to others, are rarely used at diagnostic sites because of their high price. The numbers provided below were obtained from many years of experience using gas analyzers.

Before we provide the numbers, let’s focus on the following issue.

The majority of modern petrol engines are equipped with an exhaust gas catalytic converter. Therefore, the EG compositions of such an engine and those of an engine without a converter will differ considerably. Based on this fact, it seems most reasonable to study the EG composition in the exhaust system upstream and downstream of the converter. These numbers are the reference points from which all subsequent conclusions will be made. You could say that they are the foundation of gas analysis. They must be remembered and kept in mind all the time. So,

- In the exhaust system upstream of the catalytic converter, the EG composition of a fault-free engine that has warmed up to the operating temperature and is running on the stoichiometric mixture looks as follows (Table 1):




100…200 ppm







        - In the exhaust system downstream of the converter, the EG composition of a fault-free engine that has warmed up to the operating temperature and is running on the stoichiometric mixture, with a fault-free warmed-up catalytic converter, looks as follows (Table 2):




        10…20 ppm







        The lower CO and CH values in the second case can be explained by the chemical reactions in the converter. The oxygen percentage also decreased due to its consumption in oxidation reactions. The amount of carbon dioxide CO2 increased due to CO oxidation.

        We do not see nitrogen oxides NOx here, but you should not forget that they are reduced to pure nitrogen in the converter and lose their harmful environmental impact. Please note that the λ value in both cases is equal to 1.

        The gas analysis parameters we have reviewed are reference points. They are the values that would be shown on the instrument display for a warmed-up engine running on the stoichiometric mixture and in absolutely good condition. Now let us talk about the deviations that may be encountered in practice and about EG composition analysis in those cases.

        Leaks in the exhaust system

        One should remember that gas moves in the exhaust system with complex wavelike behaviour. There are pressure zones that alternate with depression zones.

        When a leak in the exhaust system appears in a pressure zone, the exhaust gases escape with a specific sound (the system “strikes”). When a leak occurs in a depression zone, atmospheric air enters the exhaust system. Let us remember its composition. Even if the air inflow is insignificant, the O2 content in EG will increase largely as it comprises almost 21% of air but only about 0.5% of EG. At the same time, there is little CO2 in the air, so the amount of this gas in EG will change not as considerably. The same can be said about the CO and CH content.

        If air flows into the exhaust system, there is an unnaturally large amount of O2 in the EG composition. It is safe to say that the first parameter to assess during analysis of exhaust gas composition is the oxygen content. If it exceeds 1.5...2%, there is atmospheric air flowing into the exhaust system.

        It makes no sense to perform further analysis without eliminating the exhaust system defects. It should be said that a large amount of oxygen in the EG composition will also be observed when the engine misfires. But misfires typically produce a large amount of unburnt fuel, so it’s not actually possible to confuse these two situations.

        Certainly, when there is air inflow, there is no point in analyzing other EG composition parameters. In this situation, we would only notice that the calculated ratio λ takes on unreasonable values. They also indirectly indicate the fault described.

        Rich mixture

        In this case, λ < 1: there is less air in the mixture than required for full combustion. It is easy to conclude that with insufficient oxygen combustion will be incomplete and the EG will contain more CH that they would with the stoichiometric mixture. The CO content would increase for the same reason. The amount of CO2 would become less than when running on the stoichiometric mixture, because fuel has burnt in a non-optimal manner. Therefore, the EG composition of an engine running on a rich mixture without a converter looks approximately as follows (Table 3):




        300…400 ppm







        Note that when there is a catalytic converter, a slight increase in mixture richness in terms of the EG composition can be missed and not detected, but any serious deviation would fall outside the catalyzing window and the EG composition would clearly deviate from the standard. In this case, the numbers on the instrument display would be similar to the above.

        In modern engines, a rich mixture may be caused by high fuel pressure, drift in the response of the mass airflow sensor, fuel ingress through a loose diaphragm on the vacuum pressure regulator (in systems with a fuel back leak).

        The cause can be a faulty coolant temperature sensor; such a defect can be easily detected by scanner readings. Another tricky defect worth mentioning is air flowing into the exhaust system before a tripped oxygen sensor. In such a situation, atmospheric oxygen is registered by the sensor, leading to significant mixture enrichment and even a corresponding fault code.

        Another source of excess fuel in the mixture is motor oil.

        Here we shall make a short digression. The thing is that the oil film on a cylinder face is by no means of small importance in creation of the fuel-air mixture and the processes going on in the combustion chamber. If for some reason the engine has been running for a long time using an overly rich mixture or simply didn’t start on the first try, which often happens in winter, then petrol gets into the oil.

        We can assume that the unburnt petrol runs down the cylinder walls or just penetrates through piston ring gaps. In one way or another, petrol gets into the oil and we have to accept it. There are two hypotheses regarding how it then gets into the combustion chambers. Petrol vapours with crankcase gases move through the crankcase ventilation system and get mixed with air in the intake manifold. But, as practice shows, if we disconnect the crankcase ventilation hoses from the intake manifold, the mixture become insignificantly leaner. However, everything is fixed after replacing the motor oil.

        Now we can make a conclusion: fuel molecules get into the combustion chamber from the oil film on the cylinder walls. After all, the walls are lubricated by spraying and the oil film is refreshed at each piston stroke. This phenomenon should by no means delude diagnosticians: if they observe a rich mixture or an understated fuel delivery correction factor after a failed winter start, this is absolutely normal. In such a situation, it makes sense to recommend replacing the motor oil to avoid high mechanical wear of the engine or decreased oil performance.

        Lean mixture

        Such a mixture is characterized by the value of λ > 1 and an excess amount of air. It is easy to conclude that the residual oxygen in EG will increase given the excess air in the mixture. The amount of CH will change insignificantly, because one of causes for fuel vapours in EG is the flame extinguishing within confined spaces. And this does not depend on the mixture composition. The CO value would drop noticeably. First of all, this is the result of the excess oxygen and CO oxidizing to CO2. Yet, the percentage of CO2 relative to the stoichiometric mixture will drop due to the overall increase in the amount of gas. Of course, the calculated λ ratio will be above 1. The EG composition of an engine that runs on a lean mixture and is not equipped with a converter is provided below (Table 4):




        150…250 ppm







        Causes for a lean mixture in modern engines include, first of all, air flowing into the throttle-side space. There are a lot of ways this can happen: a vacuum brake booster, damage to intake manifold gaskets, wear of the throttle valve shaft-bushing couple, and aging of rubber seals in injectors and the idle air control valve. The location of such a defect can be spotted using a smoke generator.

        Apart from air inflow, a lean mixture may be caused by low fuel pressure due to fuel pump wear, clogging of the fuel filter or mainline, decreased injector performance, or incorrect readings from the mass airflow sensor.

        It is quite difficult to detect a lean mixture on an engine equipped with a catalytic converter. The problem is that, when the converter’s operation falls outside the catalyzing window towards leaner mixtures, it continues to significantly affect the EG composition. In this case, you should use the CO2 value and estimate the combustion efficiency as a whole.

        High CH content

        For an engine without a converter, the normal value of this parameter is 100...200 ppm. If the instrument display shows CH equal to 300...400 or more, this is an indication that you should look for a reason why the petrol isn’t burning at all, in other words, there are misfires.

        We can list many reasons for such misfires. Worn or faulty spark plugs, high-voltage wires, a faulty ignition module, unadjusted expansion gaps on valves, decreased compression, or a faulty injector. And all this can be in one or more cylinders. Another cause for high fuel vapour content in EG is an exhaust valve that is not tight or has started to fail. In this case, some portion of the supplied fuel is just discharged into the exhaust system during the compression stroke. The engine can still run quite normally, and other gas analysis parameters may be ok. The table below contains an example of the EG composition of an actual converter-less engine with faulty plugs (Table 5):




        384 ppm







        All other engine systems are known to be absolutely ok. Let us analyze the data obtained.

        High fuel vapour content in EG shows that fuel just isn’t burning.

        CO content below that which corresponds to the stoichiometric mixture and its value lets us conclude that the mixture is not rich.

        A high oxygen content combined with a high amount of CH leads us to assume misfiring. Where is the oxygen coming from? From the engine cylinders, which are just discharging atmospheric air mixed with fuel vapours when a misfire happens. The CO2 level is below the standard value, which also indicates abnormal combustion. And finally, there is the calculated λ ratio, which is computed by the instrument based on the oxygen content among other things. Indeed, misfires were observed in the engine in question; they were well heard near the lip of the exhaust pipe.

        In a converter-less engine, misfires are not accompanied by any special problems, except for the emission of toxic substances. But misfires on engines equipped with catalytic converts cause unacceptable warming of the engine. Unburnt fuel vapours mixed with oxygen undergo a reaction on the catalyst’s support surface, releasing a large amount of heat. The temperature of the converter’s catalyst support and housing increase to 1000°C or more. This phenomenon is very dangerous and can, for instance, ignite dry grass below the car or damage parts adjacent to the converter.

        In practice, often the interior noise insulation melts and insulation on electric wires adjacent to the chassis are destroyed and shorted.

        But most importantly, misfires and the subsequent overheating of the converter destroy the converter. Honeycomb sintering takes place in the ceramic catalyst support, causing the exhaust system’s gas-dynamic resistance to increase.

        If the substrate is made of corrugated foil, it does not typically sinter, but the active catalytic layer is destroyed and the converter stops performing its function. One way or another, misfires in an converter-equipped engine are a very dangerous phenomenon.

        Therefore, the modern-day engine control system monitors misfires and, if it detects them, cuts off the faulty cylinder.

        Analysis of the CO2 level

        As previously mentioned, this EG component is a product of the most complete combustion of fuel. The better fuel is burned in the engine cylinders (and “burned down” in the converter), the higher the amount of CO2 in the EG.

        This statement may appear to be incorrect with respect to an engine that injects fuel directly into cylinders that run on a lean mixture. But so far our concern is the more common engine with injection into the manifold. For a fault-free converter-less engine, EG contain about 14% CO2, while for a converter-equipped engine that figure is 16%. Citing these numbers, it is hard to say that these are the numbers that will be displayed by your instrument. It is better to see what readings the instrument you are using shows and use them as a reference. But this does not change the general principle of analysis.

        After obtaining the CO2 value, it should be evaluated.

        If it is approximately equal to the maximum permissible value for this engine type (please see Tables 1 and 2), we can conclude that there are no problems related to fuel delivery and the creation of the fuel-air mixture. On the contrary, a decrease in CO2 content should alarm you, because it is a sign of a problem.

        Of course, the gas analyzer will not indicate the faulty sensor or element, but it will guide your search or at least indicate the existence of such a part.

        In the author’s experience, there have been cases when the engine seemed to be running normally based on all the parameters, but the CO2 amount in EG indicated a problem. Eventually, a fault was detected and measures were taken to eliminate it. This parameter is the one that lets you assess how a converter-equipped engine is running without looking at the CO and CH values, which in this case are close to zero and carry no information.

        Monitoring the condition of the catalytic converter

        Modern-day electronic engine control units monitor the converter’s status and output a corresponding fault code if its efficiency decreases. However, it is unreasonable to condemn a rather expensive assembly to be replaced based only on a code output by the unit.

        You should make sure that the diagnosis is correct, and in this case the gas analyzer is the only instrument that can help you. The method of evaluating the converter’s performance is based on how the converter works. Since it begins to perform its function only if the temperature is high enough and if the engine is running on a stoichiometric mixture, the engine should be warmed up until the fan turns on and you should be sure that the oxygen sensor feedback loop is closed when using the scanner. Then analyse the composition of EG.

        First of all, the EG composition should be checked when the engine is idling. If the converter is fault-free, the EG composition will match the above reference composition for a converter-equipped engine (Table 2). If you observe high levels of CO (0.1%...0.6%) and CH (50…200 ppm), as well as low CO2 content, then the converter has failed.

        If there are no problems and the numbers displayed match the reference data, the speed should be increased to approximately 4000 rpms and then take gas analyzer readings once again.

        The idea behind the method is this: At a small EG flow typical of low speeds, the converter manages to fully treat harmful components. At a large flow and high speed, it may not be efficient enough. Therefore, we can only measure a converter’s performance by its ability to achieve the EG composition’s reference parameters at high speed.

        Just as a matter of interest, you can perform the following experiment. Connect the gas analyzer to the exhaust pipe of a cold engine, start the engine, and monitor the EG composition. You can clearly trace the initial engine running on the enriched mixture, then the gradual change of parameters toward the stoichiometric mixture and, finally, the shift of parameters toward the reference levels for a converter-equipped engine.

        Such experiments are very useful as they obviously connect the engine and control system operation theory with practical performance results observed using instruments.


        Operating a gas analyzer requires a creative approach.

        You cannot use any algorithms here. You need to interpret numbers on the instrument’s display critically and reflect on why they are what they are and where any given component came from.

        We have reviewed the most fundamental, basic aspects of EG composition analysis. Now it is a matter of practice and developing your own experience.