ARG engine faults.

The G62 sensor tells the engine control unit how hot the coolant is. If it is faulty, the control unit thinks a warm engine is cold, and runs it rich: hard warm or cold starting, a rough idle, markedly increased fuel consumption, and a dash temperature gauge that jumps around or never reaches its usual reading. The part is cheap and can be replaced in half an hour: the trouble is that many people do not recognise it and replace an injector or a lambda sensor instead. If the fault persists, the rich mixture can destroy the catalytic converter too.

From the late nineties to around 2006, petrol engines used a secondary air pump to blow fresh air into the exhaust on a cold start, so the catalytic converter reaches operating temperature faster. The impeller of the pump rusts or its bearing seizes, soot clogs the combination valve (N112), and the rubber hoses crack. The control unit then stores the `P0411` code. It barely affects driving, but the engine warning light is on, and the emissions test can fail because of it, and the roadworthiness test with it.