Engine diagnostics. Where to begin?

Anyone who decides to commit themselves to automotive diagnostics and repair asks this question. The most complex topic in this area is the diagnosis of state-of-the-art engines.

Major requirements for a potential auto diagnostician:

  • The desire, ability, and capacity for self-study
  • Basic, intermediate (better), or deep (ideal) knowledge of ICE (internal combustion engine) theory
  • Ability to figure out electrical equipment and freely read electrical schematics
  • Ability to use a computer, electronic databases and reference materials, and diagnostic instruments and equipment
  • Knowledge of electronics and soldering skills are encouraged
  • And, last but not least, good intuition

You must have a clear idea of the specific nature of engine diagnostics: you cannot limit yourself to any one aspect of the car, because everything in interconnected. Many problems are not directly related to the injection system. The diagnostician must perfectly know the inner workings of the engine, be a good auto electrician, and know injection systems – from both the modern-day and years past.

It is crucial for the diagnostician to set up their workplace properly. It’s certainly quite difficult to purchase everything at once, but you must have at least basic devices.

What equipment is required in a diagnostic site?

Let me say up front that I don’t find diagnostic methods based on your ear or eye to be acceptable today. I don’t wish to detract from the role of a human in the diagnostic process. To the contrary, considering the specialist to be essential, without which it would generally be impossible to achieve any noticeable result, I continue to insist that high-quality equipment is absolutely indispensable.

People are simultaneously the strongest and the weakest link of any process. They can be tired, ill, or on vacation. Somebody else must fill in for the absent specialist and continue the same job.

So we are putting together an engine diagnostic site.

All diagnostic devices can be categorized into three basic groups.

These groups are the very foundation. Without them, skilful troubleshooting turns into a dull process based on replacing old parts with new ones.

A diagnostic site absolutely must have at least one representative of these three groups:

  1. Scanners
  2. Motor testers
  3. Gas analyzers

Let us review each group in detail.


The main element of a modern engine’s control system, which complies with strict pollution regulations, is an electronic control unit (ECU).

A scanner is designed specifically to work with the ECU, i.e. to “scan” it. Let’s review how the unit works. It gets information about the engine’s current status from sensors installed on the engine, processes the engine’s status according to the installed program, and generates control signals for mechanisms known as actuators.

Additionally, the ECU detects faults in the control system. Applying a scanner to the unit, we can:

  • Monitor signals from system sensors and watch their behaviour over time
  • Inspect the operation of actuators by running them and monitoring them visually or otherwise
  • Read fault codes saved by the system
  • View identification data for the ECU, the system, etc.
  • Make required adjustments
  • Program control units

The scanner’s measurements represent what the ECU “sees”.

They are not true values of voltage or other parameters.

If sensor measurements are incorrect for any reason (e.g. a poor ground connection), then the scanner’s screen will show us incorrect data. In other words, the scanner is not a measuring instrument. It only displays data from the ECU – you must understand that and treat the information it provides accordingly.

You should treat fault codes read by a scanner with equal caution. These codes are not instructions for replacing parts. Rather, they are the basis for thought and investigation.

For example, suppose you get an oxygen sensor error, the mixture is too rich. Should we replace something? Of course not. We need to find the reason for the rich mixture.

There are essentially two scanner types: dealer scanners and multibrand scanners.

Dealerships are equipped with dealer scanners. These devices are quite expensive, but only they allow you to perform the full range of ECU operations for a particular vehicle brand.

Multibrand scanners are usually fairly good copies of the dealer devices.

Scanners can be portable devices or software-based instruments that work with the PC. Both types have their own advantages and disadvantages. Details on a specific device can be found on the manufacturer’s website.


This is a completely different kind of diagnostic equipment.

In contrast to a scanner, a motor tester is a measuring device. The information it provides is read directly from the engine and lets you detect faults that a scanner cannot.

For example, you can measure the voltage waveform and current of sensors and actuators; high-voltage traces, cylinder pressure, and fuel pressure. We’re talking about the ability to check cylinder balance and measure cranking amps, the ignition advance angle, and take many other measurements.

Where is a motor tester used?

A spark inside engine cylinders causes ignition and combustion of the fuel-air mixture. This process cannot be directly monitored and evaluated (e.g, visually). But it’s quite easy to estimate it indirectly. Motor testers enable this by providing the ability to read secondary voltage traces.

Literally everything affects the shape of these traces: the condition of the ignition coil, high-voltage wires, plug tips, plugs, compression, the valves’ condition, the mixture composition, and even the ECU’s working condition.

Another very informative graph provided by a motor tester is cylinder pressure during engine operation. To take this reading, the plug tip of the cylinder in question is connected to a spark gap, the plug is unscrewed and replaced by a pressure sensor.

The graph generated from these measurements lets you draw conclusions about the following:

  • Whether the valve train phase setting is correct. Not only does this relate to belts, but also problems such as broken crankshafts and camshaft keys and a rotated crankshaft pulley
  • The condition of the cylinder-piston group and valves
  • Whether there is air inflow to the intake system
  • High back pressure of the exhaust system (damage to the catalytic converter’s cells or the muffler baffles)
  • The actual ignition advance angle

You must admit that this list is impressive. I need only mention the proper adjustment of the phase setting. Doing this manually is difficult and time-consuming, but a motor tester solves this problem effortlessly in five minutes.

Similarly, you can detect breaks or a coil-winding short circuit in the nozzles. You can measure the cranking amps and draw conclusions about the condition of the battery and starter. The alternator’s voltage waveform also lets you determine whether it is working properly.

A motor tester lets you to check that sensors are working.

For instance, we can take an oscillogram of the signal coming from an air mass flow sensor while the supply voltage is fed to it. Based on the transient response, you can tell if the sensor works right away, without even starting the engine.

Our training videos explain in detail how to do this.

If you’re convinced of the necessity to purchase such a device, all that remains is to decide on a specific model. Unfortunately, motor testers are most expensive of the three types of instruments mentioned above. There is a wide range of companies and models to choose from. You certainly don’t need to buy an ultra-expensive brand-name motor tester, but you wouldn’t want to the simplest device either.


Two issues are crucial here.

First, a modern diagnostic site only needs to have a four-component gas analyzer. Two-component devices, such as carburettors, belong to history.

Second, a gas analyzer is meant to be a source of diagnostic information, rather than something for “adjusting СО”.

How do you use this information for engine diagnostics? Please read the article Gas Analysis and Diagnostics.

Brief summary

All three types of devices described work in entirely different ways, give us different information, and in no way replace each other.

Yes, the data they provide may overlap to some extent, but in some cases they are unique for each device. In theory, you can do without any of these devices, and there are “pros” that can do anything with only a single screwdriver. That’s not the point. The point is that competent troubleshooting is based on information analysis – on measurements. And as is widely known, that’s where the science begins.

Other equipment is supplementary, though having it is more than desirable:

  • Fuel pressure gauge
  • Ultrasonic or liquid-type injector cleaner with an inspection bench (a very useful thing)
  • Inspection benches for spark plugs and ignition modules
  • High-quality multimeter, preferably a model specially designed for working on engines
  • Good tool kit, preferably a name-brand kit
  • Testers of all kinds and clever devices made by specialists themselves

Finally, the last thing a diagnostic site cannot do without is information.

A specialist must get it by all available means: the Internet, books, and publications in automotive magazines.

You must also have continuously updated databases: Autodata, Mitchell, etc.


The diagnostician’s work consists of three stages: collection of diagnostic information processing of diagnostic information, and decision-making.

All the equipment listed above is used to collection information. The actual process can be described as follows.

1. Ask the customer what the problem is. When, how, and under what circumstances does the problem reveal itself? “Enhanced interrogation” techniques often make the subsequent investigation considerably easier.

2. Visually inspect under the hood. Carefully look for visual damage to electrical wiring, hoses and high-voltage wires. Is there evidence that someone, most often an installer of gas equipment or a car alarm system, has been messing with things. It is helpful to make sure that all ventilation hoses for the crankcase, adsorber, etc. are in their proper places, EEC fuses are not burnt, and there is petrol in the tank. Inspecting the condition of the air filter is highly desirable. Often times it is torn, resulting in a MAF sensor failure. Only after all this has been done should you start working with instruments.

3. The first thing to do is use a scanner to find out what type of ECU and system you are dealing with. Remember specific features of its operation, its components and possible “congenital defects”. At this stage, you also need to measure compression in the cylinders to immediately determine whether any deeper interference in the engine is required. If the compression is low or varies greatly among the cylinders, a visit to a mechanic will be necessary.

4. Visually inspect the plugs. Look at the amount of carbon, the colour, the gap, electrode state, and whether the insulation is punctured. Unfortunately, the only assistants in this operation are experience and intuition.

5. Use the scanner to check readings for sensors and actuators. You can move the idle air control valve, turn on the fan and petrol pump, and test the injector balance.

6. Perform diagnostics on the supply system based on the fuel pressure. If there are no problems with the pump, pressure regulator, sensors, actuators, plugs and wires in statics, then start the engine.

7. Check the same parameters on the running engine using the scanner. You also need experience here; this process can’t be explained in a few words. Listen to the engine carefully to detect unwanted sounds, knocks, and humming.

8. Record the gas analyzer’s readings.

9. If required, take high-voltage oscillograms.

10. If you suspect that the valve train’s phase setting is incorrect, then check the in-cylinder pressure with the motor tester.

11. And now we are coming to the most interesting thing. Carefully look at the results obtained, analyze them, and draw conclusions.

Sometimes in questionable cases it makes sense to replace a faulty part and take readings again, or to take a test ride. To do this, the diagnostician’s workplace should be provided with a stock of replacement parts.

In any case, you should strive to attain such skill that you can almost always find faults using instruments alone.