How to check lambda sensor?

I was led to write this material by abundant questions on an online forum, which come from incomprehension (or misunderstanding) of how an oxygen sensor, or lambda probe, works.


First of all, you should move from the general to the specific and understand how the system operates as a whole. Only then you will have a proper understanding of how this rather important element in the engine control system works and diagnostic methods will become clear.

To avoid getting bogged down and overloading the reader with information, I will talk about the zirconia lambda probe used on most inexpensive cars. Those who wish to study the subject in a deeper way can find and read materials on titania sensors, wideband oxygen sensors (WOS), and think up ways to test them on their own. We are going to talk about the most common sensor known to most diagnosticians.

Sometime a long while ago, the lambda sensor was just a sensing element without any heater. The sensor was heated by exhaust gases, and this took a lot of time. Stringent pollution regulations required that the sensor quickly start working to its full extent, so the lambda probe acquired a built-in heater. Therefore, a modern lambda sensor has 4 outputs: two of them are the heater, one is ground, and the other one is signal.

Of all these outputs, we are only interested in the signal.

You can view its voltage waveform in two ways:

  • With a scanner
  • With a motor tester, by connecting the probe and turning on the recorder

The second option is preferable. Why? Because the motor tester lets you evaluate not only current and peak values, but the signal waveform and change rate as well. The change rate is what describes the sensor’s good condition.

So here’s the main thing: the lambda sensor responds to oxygen. Not to the mixture composition. Not to the ignition advance angle. Not to something else. Only to oxygen. You must realize that.

The physical principle of the sensor’s operation is dealt with in many books dedicated to electronic engine control systems, and we are not going to dwell on it.

A reference voltage of 0.45 V is supplied from the ECU to the sensor’s signal output. To convince yourself, you can switch off the sensor connector and check this voltage using the multimeter or the scanner. Is everything as expected? Then reconnect the sensor.

By the way, the reference voltage on old cars “floats”, negatively impacting the normal operation of the probe and the entire systems. Most often, the reference voltage with the disconnected sensor exceeds the required 0.45 V. The problem can be solved by selecting and installing a resistor pulling the voltage to the ground, thereby returning the reference voltage to the level required.

Beyond that, the way the sensor works is simple. If the amount of oxygen in gases flowing around the sensor is large, the voltage on it will drop below the reference level 0.45 V, to approximately 0.1 V. If the amount of oxygen is low, the voltage will get higher, about 0.8-0.9 V. The beauty of the zirconia sensor is that it “jumps” from low voltage to high voltage at oxygen levels in exhaust gases in a way that corresponds to the stoichiometric mixture. This remarkable feature is used to keep the mixture composition at the stoichiometric level.


Once you understand how the lambda sensor works, it is easy to understand how you might inspect it.

Let us assume that the ECU returns an error related to this sensor, e.g. Р0131 “Low signal from oxygen sensor 1”. You need to understand that the sensor reflects the system’s condition and, if the mixture is really lean, it will show it. Replacing it is absolutely pointless.

How can we determine whether the problem is in the sensor or in the system? It is very simple. Let us model one situation or another.

  • For instance, when there is a complaint about a lean mixture and low voltage at the sensor’s signal output, we increase fuel supply by pinching the leak-off hose or, if there is none, by injecting petrol from a syringe into the intake manifold. How does the sensor respond? Does it show a richer mixture? If yes, then there is no point in replacing it – you need to find out why the system is supplying an insufficient amount of fuel.
  • If the mixture is rich and the probe shows it, try to introduce an artificial air inflow by removing some vacuum hose. Did the voltage on the sensor drop? If so, then the sensor is working just fine.
  • A third option (quite rare, but it has its place) is to introduce air inflow and pinch the leak-off hose. Either the sensor’s signal does not change (remaining at 0.45 V) or it changes (but very slowly and within a small range). That’s it – the sensor’s dead, since it must be responsive to mixture composition variations to change the voltage at the signal output quickly.

For a deeper understanding, I’m going to add that when you have a little experience it is easy to establish how worn out the sensor is. This can be done based on the slope of the transition from the rich mixture to the lean one and back. A good, working sensor responds quickly, with a nearly vertical transition (of course, this inspection is performed with the motor tester). A bad or simply worn-out sensor responds slowly with smooth transition edges. Such a sensor must be replaced.

Understanding that the sensor responses to oxygen, you can easily solve another common issue. With misfires, when the atmospheric air and petrol mixture is discharged from the cylinder to the exhaust systems, the lambda probe will response to a small amount of oxygen contained in this mixture. Thus, an error indicating a lean fuel-air mixture is very likely if the engine is misfiring.

I’d like to draw your attention to another important issue: the possibility that there is air flowing into the intake system somewhere upstream of the lambda probe.

We mentioned that the sensor responded to oxygen. What would happen if there is a crack upstream of it at the exhaust? The sensor will respond to the large amount of oxygen, which is equivalent to a lean mixture.

Please note that it is exactly equivalent.

However, the mixture can be (and will be) rich, and the probe’s signal will be mistakenly perceived by the system as an indication of the presence of the lean mixture. And the ECU will make it even richer! As a result, we will have a paradoxical situation: a “lean mixture” error but a gas analyzer showing that the mixture is actually rich. By the way, this is a case where a gas analyzer is a very good assistant to the diagnostician.

How to use information obtained using this device is described in the article Gas Analysis and Diagnostics.


  • You need to clearly distinguish between faults in the electronic engine control system and faults in the lambda probe.
  • The probe can be checked by monitoring the voltage at its signal output using the scanner or by connecting the motor tester to the signal output.
  • By artificially simulating a lean or rich mixture and observing the probe’s response, you can draw a reliable conclusion about its working condition.
  • The slope of the voltage transition from the “rich” condition to the “lean” condition and vice versa lets you easily draw a conclusion about the lambda probe’s condition and remaining life.
  • An error indicating a lambda probe fault is by no means a reason to replace it.