The first step is to carry out a visual inspection of the sensor body, lead and connector. For personal safety this should be done with the engine switched off and ensure that the engine and components are cool enough to touch.

On this page are some typical examples of sensors that have sustained physical damage and would require renewal. For heated lambda sensors (will have 3 or more wires) another simple test is to check the integrity of the heating element. This can be done using a suitable ohmmeter. Great care must be taken to identify the correct heater element wires. A reading greater than 30 Ω means that the heater is defective and the sensor should be renewed. A reading of infinity ∞ means that the heater coil or wiring is broken; again the sensor must be renewed. Refer to the relevant section for wiring colours.

For Zirconia Binary sensors

Reference to the relevant workshop manual should be made prior to testing. Ideally on the vehicle sensor testing should be carried out using an oscilloscope connected between the sensor and vehicle using an appropriate harness.

As the sensor only functions correctly once it has reached approximately 350°C it is essential that the vehicle is brought up to normal working temperature before checking the readings. Once the engine and sensor have reached normal working temperature raise the engine speed to around 2000 rev/min, this should produce a fluctuation in voltage from approximately 0.2 volt to 0.8 volt as the sensor switches from fuel lean to fuel rich. This switching should occur in approximately 300 milliseconds. The next part of the cycle (fuel rich to fuel lean) should have a similar reaction time. Deviation from these figures will require the sensor to be renewed; a slow switching time is a fault that many Lambda diagnostic tools are incapable of measuring.

A visual inspection of the protection tube when the sensor has been removed can provide valuable clues about conditions in the combustion chamber.

Lead
Shiny deposits are evidence of lead in the fuel. Lead attacks the precious metal of the sensor element and the catalytic converter. The sensor needs to be replaced. After changing the sensor, care should be taken to use only lead-free fuels.

Soot
Thick soot deposits lead to blockage of the sensor protection tube and have a negative effect on reaction time. Causes can be a mixture that is too fuel-rich or the result of damage to the sensor heater. The sensor must be replaced in all cases.

Oil
Thick white or grey deposits are evidence of the use of fuel additives or that the engine is burning oil. Certain components in the fuel additives and the oil contaminate the sensor element. The cause must be removed and the sensor be replaced by a new sensor.

It must be stressed that this testing process does not isolate the sensor from the rest of the vehicle’s systems and a fault in another part of the engine, its control system, the exhaust system or fuel used may affect the performance of the sensor. Equipment in off-vehicle testing uses special heated reference gas and is regularly calibrated.

For Titania sensors

As the Titania sensor operates in a similar way to the Zirconia type, the signal characteristic will also be similar but possibly with greater amplitude (depending on the control strategy). Testing can be more difficult and great care must be taken, always referring to the vehicle manufacturer’s data and test procedures.

For ZFAS-U type (Air/Fuel Sensor)

Due to the very different way in which these sensors work and the risk of possible damaged to the sensor and associated systems the only test we recommend (other than visual inspection) is to check the heater circuit resistance.

For down-stream, catalyst monitoring sensors

These sensors will usually be of the lambda 1.0, binary switching type. When the exhaust gas treatment system is functioning correctly the output signal will have significantly reduced amplitude. If the amplitude exceeds a certain threshold the management system will identify a potential problem with the catalyst or associated components. Always refer to the vehicle manufacturer’s data and test procedures.

OBD (On Board Diagnostics)

Most modern vehicles are equipped with these diagnostic systems and stored fault codes can be retrieved from the vehicles electronic memory. These fault codes refer to various components on the vehicle and suggest a fault has occurred with the component or associated components.

Don’t forget to consider the reasons why the sensor appears to have failed. It might simply be at the end of its service life but consider other possibilities including a coolant leak into the combustion chamber which can lead to anti-freeze reaching the sensor and contaminating it; the solvents of some additional gasket sealing materials used around the engine can also be detrimental to the sensor. Vibration from an unsecured exhaust will shorten the life of a sensor.