Engine Management Light Diagnosis

The engine management light, also known as the engine warning light or EML, means the engine control unit has stored a fault code. We confirm the cause on almost every make and model before any part is replaced.

The correct technical name for the symbol is the malfunction indicator lamp. The engine control unit switches it on when a sensor reading, a misfire count or an emissions value moves outside the limits set by the manufacturer. Drivers and handbooks from outside the UK often call it the check engine light. It is the same symbol and the same system.

Steady amber light. A fault has been stored and the vehicle should be inspected soon. If the car is driving normally, it can usually be driven gently to the workshop.

Flashing light. This normally indicates an active misfire severe enough to overheat and damage the catalytic converter. Reduce speed, stop when it is safe to do so and call us before driving further.

Red light. Some manufacturers display the engine warning in red when the control unit classes the fault as serious. Stop when safe and refer to the vehicle handbook before continuing.

Light that comes on and off. This points to an intermittent fault. The code remains stored in the control unit after the light goes out, so it can still be read and investigated.

The light covers a wide range of faults. Those we see most often are faulty oxygen or mass air flow sensors, misfires from spark plugs, coils or injectors, sticking EGR valves, boost leaks, vacuum leaks and wiring faults. On diesel engines, blocked particulate filters and glow plug circuit faults are also common causes of the engine management light. Some are inexpensive to correct and some are not, which is why the cause is confirmed before any money is spent on parts.

A code reader will display a fault code and switch the light off in seconds. The fault itself remains and the light will return. The code also only identifies which circuit reported the problem. A lambda sensor code, for example, can be caused by an air leak or a fuelling fault rather than by the sensor.

We read the codes from every control module, then review the freeze frame data and live data to see what the engine was doing when the fault was stored. From there we test the circuit or component the data points to, using a Picoscope oscilloscope, a smoke leak tester and manufacturer software such as ODIS, ISTA and XENTRY where it applies. Once the cause is confirmed, you receive a written explanation and a quote. No work is carried out without your approval.

An Advanced Diagnostic Assessment costs from £135. It covers reading every control module, analysing the stored data and testing the components involved until the cause is confirmed. The repair is quoted separately once the fault is known.