Top Tips: Making sense of Oxygen Sensors

What is an Oxygen or Lambda sensor?

Oxygen (or Lambda) sensors are key to the running of a modern petrol engine. They monitor the exhaust gases and feed information back to the engine management system to allow it to adjust the fuel mixture and keep the engine running smoothly.

The sensors are fitted into the exhaust - generally one at the front of the exhaust and one near the back. The front (or pre-cat) sensors provide the information on which the car can base its fuelling decisions, whereas the back (or post-cat) sensors confirm that the catalytic converter is working efficiently.

How do you know if you have a fault?

An oxygen sensor will normally work at best performance for approximately 60-70,000 miles.

If it starts to fail or is damaged, the readings provided to the engine become inaccurate and the fuel mixture is then incorrectly calculated - often leading to overfuelling.

Oxygen sensor failure can be apparent in a number of ways:

High emissions on an MOT test

Engine Warning light on a dashboard

Poor idling or stalling

Using test equipment

Test equipment is a great way to determine the cause of an engine warning light or running problem. The tools, such as our GCR05, extract the fault code from the car and provide clues as to the cause of the problem itself, for example, if an oxygen sensor fault is reported, it might be showing:

a circuit fault - referring to the wiring of the sensor, rather than the sensor itself. Often caused by wires becoming loose or damaged.

a heater fault - referring to a fault within the heater element inside the sensor. This isn't serviceable, so the sensor would need to be replaced.

a range/performance fault or 'lack of switching' - referring to a fault with the sensor itself. This normally indicates a sensor reaching the end of its life or having been contaminated. As sensors can't easily be cleaned, this sensor would normally need to be replaced.

Advanced test equipment

In addition to a fault code, the more advanced test equipment, such as our EngineCheck Pro system, can also display readings from the sensor itself.

In the case of the most common type of sensor (Zirconia narrow-band), the sensor voltage on a warm idling engine should produce a smooth oscillating wave between 0 and 1 volt. The frequency of this wave should increase as the car accelerates. This oscillation is also referred to as 'switching' (as mentioned in fault codes discussion above).

Replacing a sensor

Replacement oxygen sensors are available in the aftermarket for most modern cars - our online application list for sensors contains hundreds of direct-fit parts.

These are often very quick and easy to fit - normally requiring just a 22m open-ended spanner. For tricky to reach or seized sensors, there's also a specially designed socket that'll make it much easier.