The EGR cooler cools the recirculated exhaust gas before it enters the intake again, and coolant flows through it while it does so. The temperature difference between the two sides is large, so the wall can crack or the gasket can let through, and then the coolant reaches the exhaust or the intake side. This leaves no puddle under the car and the place of the leak is not visible from outside: the coolant evaporates and the level drops slowly, over weeks. The accompanying signs are whitish smoke, a rising coolant temperature, raised pressure in the cooling system and a sweetish smell. If it goes on for months the cylinder head can be damaged too, because the cooling gradually gets worse. On the EA189 engines the fault code P2425 usually comes with it. The problem is reported above 100,000 km, and replacing the cooler costs 600 to 1,500 euro.

On these engines the oil pump is driven by its own small toothed belt, and that belt runs in the hot engine oil. This is not the large timing belt that drives the camshafts, it is a separate belt in the lower part of the engine. In the oil the rubber hardens over time, its surface starts to crumble, and the loose particles block the suction side of the oil pump, so the oil pressure drops. If the belt snaps, the oil pump stops at once and the engine is left without lubrication within seconds. The rubber crumbs can also ruin the turbocharger and the camshaft adjuster, because they are lubricated from the same oil circuit. The maker specifies no replacement interval for this belt, so most cars still carry the one fitted at the factory, and the failures are reported above 150,000 km. Oil outside the specification and a stretched oil change speed the process up, because this belt ages in the oil. Replacing the belt itself costs between 200 and 600 euro, as long as the engine has not run without oil pressure.

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 four groups of parts in the air suspension (compressor, valve block, ride height sensors, air springs) work together, and any weak link causes a fault message, an uneven ride height or a car that settles. The characteristic symptom: the car sits down on one corner overnight, levels itself for minutes in the morning, and the compressor runs long and loud. According to the reports the compressor lasts 80,000 to 100,000 km; and a leaking air spring takes the compressor with it, because it keeps trying to make up the loss.

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.

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.