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Modern cars have a multitude of sensors feeding information to the electronic control unit which coordinates and regulates all engine systems.
An important one is the exhaust gas oxygen sensor, also called the lambda sensor, fitted to cars with a catalytic converter.
It is typically screwed into a threaded hole in the exhaust pipe, near to the manifold. Its probe protrudes into the pipe where the exhaust gases swirl around it when the engine is running.
In the tip of the probe is a piece of high-temperature ceramic, coated with a very thin layer of platinum. The job of this sensor is to monitor the amount of oxygen present in the vehicle's exhaust gas.
This provides an accurate indication of how close the air:fuel ratio entering the cylinders is to the ideal ratio of 14,7:1. The catalytic converter will only work properly if the air:fuel ratio is maintained within very tight limits around this ratio.
The control unit achieves this by continuously making tiny corrections to the amount of fuel injected, in response to information received from the oxygen sensor.
The first oxygen sensors, which had only a single wire coming out of the sensor body - the wire that must carry the signal to the control unit - had a weakness. And that was, when the engine had been started when cold, the probe took too long to reach the temperature of 300°C which is required before an oxygen sensor starts to generate a signal.
Furthermore the tip temperature could fluctuate during normal operation, from about 400°C with the engine idling to about 900°C when the engine was working hard.
In due course a small electrical heating element was incorporated in the probe to warm up the tip faster and smooth out the temperature swings.
Heated sensors can be recognised by the feature that they have three or four wires coming out of the housing. The first generation of these heated sensors appeared in the mid-1980s and were fitted in production for many years after that. Recently, a second generation of heated sensors became available which have a longer service life.
Though steadily improving, oxygen sensors are not yet at the point where they will last the lifetime of the engine.
Like spark plugs, they still have to be replaced periodically. The most common cause of sensor failure is a build-up of deposits on the probe tip. These can be oily deposits if the engine has an oil consumption problem, lead deposits if leaded fuel has been used in the engine at some stage, or silicone deposits if the wrong kind of silicone sealant has been used, for instance to seal up a leaky manifold gasket - always look for the "sensor safe" label on silicone sealer - or if antifreeze containing silicates leaked past a head gasket.
Or it can simply be a gradual build-up of carbon deposits, especially if the car is constantly used for short trips.
Any of these deposits will make the sensor sluggish to react to changes in oxygen content and will weaken the signal that it sends out.
The control unit interprets this as a sign that the engine needs more fuel and accordingly it sees to it that more fuel is injected. The results of the rich fuel mixture and a sluggish oxygen sensor are decreased fuel economy, hesitation on acceleration, stalling, surging, rough idle and premature failure of the catalytic converter.
The same symptoms will be experienced if a perfectly healthy oxygen sensor detects a "lean mixture" condition (too high oxygen content in the exhaust gas), perhaps because of an air leak in the inlet or exhaust manifold or even a fouled spark plug.
The oxygen sensor has no way of knowing where the extra oxygen came from. The sensor should therefore be tested, preferably by watching its output signal on special test equipment, before it is blamed for driveability problems.
Generally the oxygen sensor should be replaced at intervals of between 50000 and 150000km, depending on sensor type, engine condition and vehicle usage.