Mercedes-Benz OM656

The straight-six diesel that replaced Mercedes' OM642 V6, sharing its modular architecture with the petrol M256: 210 to 270 kW, 600 to 750 N·m, twin turbochargers and on some versions an electrically driven supercharger.

Mechanically it has very little wrong with it, which is worth saying as plainly as the opposite would be. It has none of the timing-chain reputation of the diesels of the previous decade, and where it is reported as failing the cause is almost always the emissions equipment rather than the engine.

So the fault picture is the AdBlue system and the particulate filter, and both are diseases of how the car is used rather than of how it was built. A filter that never reaches temperature because every journey is four cold miles will block, and no repair to the engine prevents it. Before condemning anything on this engine, establish what kind of journeys it does — the answer usually is the diagnosis.

Turbocharger failure and oil leaks make up most of the remainder.