AUM engine faults.

This is the engine that made the VW-Audi oil sludge affair famous. The oil capacity is tight to begin with (just 3.5 litres in the longitudinal engine B5 Passat and A4), the turbo returns hot oil, and the extended "longlife" service interval cannot take it: the oil thickens, congeals into sludge, and blocks the screen of the oil pump. The symptoms are low oil pressure, the warning light, slow draining oil, greasy deposits at the filler neck and on the filter, and finally limp mode. VW acknowledged the fault in 2004, gave an extended warranty, and even reimbursed repairs already paid for. The clean-up is a substantial workshop bill; and if the pump has already stopped delivering, the engine pays for it.

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.