After standing for a longer period, on a cold start you hear a rattling, knocking noise from the timing chain for a few seconds. The cause is oil pressure draining away overnight: the chain tensioner can only tension properly once the oil pressure has built up. A slack chain causes misfires and loss of power, and if it jumps a tooth the engine is badly damaged.

On short urban trips the particulate filter cannot burn through. The engine control unit then injects extra fuel late in the cycle to heat the filter, and part of that fuel passes down the cylinder wall into the sump. The oil level rises, the oil thins out, and it lubricates worse.

The most common source of faults on the longitudinal 7 speed S tronic that replaced the DL501 (0CJ / 0CK / 0CL) is the mechatronics unit as well. Typical faults are the input or turbine speed sensor (which makes the shifting unpredictable), clutch slip under load, and abnormal operation of the park lock.

On AdBlue (SCR) diesels the NOx sensor after the catalyst ages, and the tip of the dosing valve crystallises. An AdBlue warning appears on the dash, often with a countdown ("X km of starts remaining"), while the tank is full. Crystallisation typically forms with short trip, urban use, or from poor quality AdBlue; on a car standing below freezing the circuit can freeze up too.

The internal springs of the flywheel tire, and the two discs knock against each other. The signs are a rattling or knocking from the bellhousing at idle and when releasing the clutch pedal, and vibration through the floor. If the clutch slips without a burnt smell, that also points to the flywheel. It is worth replacing in time: a disintegrated flywheel can destroy the bellhousing too. At a clutch replacement the two go together anyway.

The vanes of the variable geometry turbo are glued together by soot and oil arriving through the EGR. First you get overboost or a lack of boost, then loss of power above 2,000 rpm, and finally limp mode. Caught in time it is enough to remove the turbo, clean it and fit a new actuator rod. Short urban trips and gentle, low rev driving speed up the deposits.

On the infotainment of the B9 platform (A4 B9, A5 F5, Q5 FY) the most common complaint is a black screen, a unit that restarts on its own, and a boot that stops partway. A significant part of these are software in origin, not a hardware fault: an update installed at a franchised dealer resolves 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.