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Ask the Expert: Why does my Mercedes keep going into limp mode with no warning lights?

Mercedes GLC
Our reader's issue with their Mercedes GLC 250 going into limp mode could be down to a problem with the fuel system - Mercedes-Benz

Dear Alex,

My Mercedes GLC 250 lost all power on a French motorway. The engine did not cut out, but went into limp mode. Mercedes-Benz told me to switch it off then on again. The problem went away, but cropped up again a few days later, at which point I tried a dealer. However, a diagnostic check revealed no fault codes. It was suggested the car could be sensitive to French supermarket fuel, however the problem has also occurred in the UK. Any ideas?

RM

Dear RM,

It is rare, but not unheard of, that some cars will go into limp mode without generating a fault code. This happens because the ECU control unit can sometimes detect an issue with a component somewhere within the electronics – but the reading is not far enough outside of the “normal” range to log a fault code and trigger the engine management warning light.

This makes tracing such problems extremely difficult. The only way to do so is to get a Mercedes specialist to plug in a computer and sift through the engine data when the fault occurs, to determine which reading might not seem quite right – an arduous task and one that must be performed before the car is switched off, at which point it resets itself.

Try to work out any other common circumstances between the failures, which you have already started; each time, it’s happened after filling with fuel from a supplier you wouldn’t normally use.

That suggests the fault is in the fuel system – it could even be an injector that doesn’t quite perform to scratch with lower quality fuel.

Short of going through all of those components, however, and replacing them one by one in the hope the fault is cured – an expensive and frankly inadvisable exercise – the only thing to do is to use the best quality fuel available.

Fingers crossed the problem won’t get any worse. If it does, the silver lining should be that the failing component will hopefully generate enough of an error to get the ECU’s attention and cause it to log a fault code.


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