The symptom is a drivetrain malfunction message on the dashboard, the engine management light, uneven running, difficult starting and limp mode. A camshaft or crankshaft timing code appears in the fault memory, and the two together are the most telling. If the timing moves far, the valve and the piston can meet, and that is internal engine damage.

Not every example is affected: many engines run without trouble throughout. The risk is higher on tuned and track-driven cars, because the load there is greater, but standard condition does not rule it out either. The fault occurs from 60 000 kilometres.

The solution is a pinned conversion: precisely drilled pins give a mechanical connection in place of the friction. The part is around 800 pounds, and 1950 pounds with fitting. One thing is worth knowing: this fault belongs to the S55 engine, and the N55, the B58 and the S58 are not affected.

The cause is often the steering rack's own control unit or the torque sensor. If the control unit does not communicate, diagnostics often returns no fault code at all, so "no fault code, so nothing is wrong" is wrong. If the assistance drops out while driving, the steering goes heavy, and you should have it checked the same day.

This is not wear but an impact, so the mileage says little here: it is reported from 40 000 kilometres, but it can happen at any time. There is no advance sign: a slow leak does not start, instead the oil pressure warning and the streak of oil under the car come suddenly. At that point you have to stop immediately.

There are two directions for the repair. There is an aftermarket cooler that comes with a removable and replaceable rock guard. The other direction is stronger construction: a bar-and-plate aluminium cooler stands up better to flying gravel, and goes into the same factory location. The suppliers themselves argue that the factory part is easily damaged.

The symptoms are knocking from the engine block, a metallic sheen in the oil and the oil pressure warning light. If a bearing runs, the crankshaft is damaged too and the repair is an engine rebuild. The bearing shell size and clearance are the same on the N54 and the N55, with the same factory part numbers. The S55, however, manages the oil pressure better, so the trouble is rarer on it than on its smaller siblings.

The sensor is usually the first to give up: oil leaks past its seal into the connector, and the oil corrupts the signal the engine control unit depends on. From then on the control unit gets no reliable position feedback, cannot set the valve lift precisely, and switches to a protective strategy. The fault code in this case is a plausibility fault, 2A67 or 2A6B: the control unit does not consider the position signal credible.

The eccentric shaft itself can also wear out or seize, and there are several causes for that: lack of lubrication from oil sludge and skipped oil changes, wear of the bearings, a system driven on with a failed actuator motor, and overheating and oil contamination in the cylinder head. It is reported from 150 000 kilometres.

The symptom is similar in all three cases: an unstable idle, stumbling and long cranking at a cold start, a check engine light that comes on and goes out on its own, lack of power or limp mode, inconsistent throttle response, and misfires cylinder by cylinder. A clicking, binding noise from the engine bay points to a seizing shaft.

The misdiagnosis is common, because the symptom shows up at combustion, so coils and spark plugs are suspected first, while the fault is in the air control. The condition of the shaft can only be established after the valve cover has been removed and it has been inspected. The sensor connector, though, is worth opening up first: if it is oily, that is the far cheaper route.