In TECHMAIL, we answer your motoring queries…

I own a Ford Fiesta 1,6 Titanium. The engine-malfunction warning light illuminates almost every day and the car goes into “safe” or “limp” mode. If I stop the engine and start it again, everything returns to normal. This has been happening for a few months and I took in the vehicle for a diagnostics check numerous times, but the tests could not pick up anything conclusive. The technician removed the throttle body and cleaned it, yet the problem persists. THARSU KISTA

Via email

The engine-check lamp (or malfunction indicator lamp; MIL) is part of the onboard diagnostic (OBD) system of the vehicle and is linked to faulty sensors, actuators or systems that negatively influence the emission levels of the engine. Limp-home mode is normally activated when the OBD system needs to protect the engine from possible damage.

We cannot understand why the diagnostic checks did not reveal what the problem is, as the engine light will illuminate only if a code is also logged in the memory of the engine control unit. This code (called the diagnostic trouble code) is linked to a sensor, actuator or system that is to blame. As you did not mention whether it’s a turbodiesel or petrol Fiesta, we’ll give you both possibilities:

In a petrol vehicle, the MIL can be linked to:
•    Lambda (oxygen sensors);
•    Throttle;
•    Injectors.

In a turbodiesel engine, the MIL can be linked to:
•    Exhaust-gas recirculation valve (EGR);
•    Turbocharger;
•    Injectors;
•    Fuel pump.

The reason the problem “disappears” when you switch off the engine is that the diagnostic needs to run again in the next drive cycle to confirm the fault. As it’s an intermittent problem, the cause might be something as simple as a small boost leak if it is a turbodiesel engine or a faulty lambda sensor in a petrol engine. Take the vehicle to a dealer with proper diagnostic equipment and knowledge.