Engine Management Light On?
Find Out What’s Wrong

The lambda sensor (also called O2 sensor) monitors exhaust gases to ensure the engine is burning fuel efficiently. It is one of the most common causes of an engine management light. A faulty lambda sensor causes increased fuel consumption, failed emissions tests and can damage the catalytic converter if left too long. There are typically two sensors — upstream and downstream — and fault codes will identify which one has failed.

The mass airflow sensor measures the air entering the engine. When it gives incorrect readings the ECU miscalculates fuel delivery — triggering the engine management light along with symptoms of poor performance and economy. Cleaning the MAF sensor with specialist cleaner resolves around 30% of cases. Otherwise the sensor requires replacement.

The downstream lambda sensor monitors catalytic converter efficiency. When converter efficiency drops below the threshold the ECU triggers an EML. Catalytic converter failure is often caused by a preceding fault — a misfiring engine, burning oil or a failed lambda sensor — that has damaged the converter. The underlying cause must be fixed before fitting a new converter.

One of the most common causes of an engine management light — particularly if accompanied by rough running or hesitation. The ECU monitors combustion events on each cylinder and stores a fault code identifying which cylinder is misfiring. Causes include worn spark plugs, a failed coilpack or a faulty injector.

A loose or cracked fuel cap allows fuel vapour to escape from the tank — triggering a fault in the evaporative emissions system and illuminating the EML. This is one of the simplest and cheapest causes to check. Remove and refit the fuel cap firmly and check it for cracks. The light may take a day or two to clear on its own.