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The sensor that tells the engine what kind of fuel is in the tank has failed or drifted.

It exists because the fuel in a modern tank is not a fixed substance. On a flex-fuel vehicle it may be anything from pure petrol to 85% ethanol, and the two need substantially different injection quantities and ignition timing — ethanol carries less energy per litre but tolerates far more compression. On a diesel the same class of sensor detects whether the fuel is diesel at all.

That second job is the one with the frightening failure mode. Diesel lubricates the high-pressure pump; petrol does not. A misfuelled diesel that is drained before starting costs very little. Started and driven, it commonly means a new pump, new injectors, new rails and new lines, because the pump destroys itself and sends metal debris through the whole system that cannot be flushed out.

So a fault here usually removes a protection rather than causing a symptom. The car runs as before; what has gone is the warning that would have caught a misfuelling in time.

On flex-fuel vehicles the symptom is real but seasonal-looking: the engine adapts to the wrong blend, so it runs poorly on one tank and acceptably on the next, which gets blamed on fuel quality rather than on the sensor reading it.