Fuel contamination riddle continues - Lambda to the slaughter

The news stories about dodgy fuel damaging cars have mentioned a mysterious device called an oxygen sensor, but what is it and how can it fail? Here's the What Car? guide.

What does it do?
The oxygen, or Lambda, sensor is a vital part of your car's electronic management system, because it dictates the ratio of fuel and air entering the engine.

Most modern cars fitted with electronic fuel injection have an oxygen sensor, and the car's electronic brain uses it to analyse the amount of oxygen in a car's exhaust gases. This tells the computer how much fuel to inject into the engine.

However, if the sensor stops working or becomes less accurate, the brain believes the engine is running 'lean' - it has a lower ratio of petrol in the fuel/air mixture - and compensates by increasing the amount of fuel injected. This causes the car to consume more fuel than necessary, run rough or lose power, and even fail an MoT emissions tests.

Does it last long?
Like spark plugs, oxygen sensors need replacing; and typically they last between 30,000 and 50,000 miles. They usually cost from £100, but they also need to be fitted by a trained mechanic. The sensor is usually located underneath the car in the exhaust system and generally isn't very accessible.

It needs to be replaced because it spends its life operating in hot exhaust gases from the engine. Over time, the pollution in the gases build up on the tip of the sensor and decrease its ability to measure oxygen.

Certain chemicals, such as lead or silicon, can quickly damage the sensor and mean it will have to be replaced immediately. The recent stories about sensors failing would indicate that something in the fuel is damaging them beyond repair. However, the cause of the problem has yet to be found.

Over time, a sensor's efficiency will decrease and stunt a car's economy and performance. Faulty sensors are a major cause of cars with poor fuel economy. Garages should check the Lambda sensor according to the manufacturer's guidelines.