تعبئة وقود خاطئ

The wrong fuel put in the tank. Petrol in a diesel is the damaging direction — diesel lubricates the high pressure pump and petrol does not, so the pump can be destroyed within a single journey, shedding metal through the whole fuel system.

Do not start the engine, which is the single most useful thing to say: a vehicle misfuelled and not started usually needs only a drain and flush, while one driven may need the pump, the injectors, the rail and the lines. Diesel in a petrol engine is less destructive but very obvious — heavy white smoke, severe misfire and a refusal to run. The timing makes it self-evident where the driver knows, and the tell where they do not is a fault that begins immediately after a fill.