The more serious trouble is at the connector. Through it the engine oil gets into the engine wiring harness and travels under the insulation by capillary action, so it reaches the engine control unit and other important parts as well. The loss of the control unit is the end of that road, and serious engine damage can follow; alongside the contaminated harness the most varied fault messages can appear. Cases have been reported where the oil ran out of the connector, and the consequential damage after that was already large.

The check is simple: the connector of the solenoid has to be taken off and looked at to see whether it is oily. In the fault memory a faulty solenoid gives one of the codes 0059, 0060, 0063, 0064, 0271, 0272, 0275 and 0276. One thing is worth knowing: according to the maker only the solenoid that has a corresponding code has to be replaced, so if a workshop has changed all four of them alongside two codes, that is worth asking about.

The engine number decides what is in the car. Above engine number 2729..30 759427 (M272) and 2739..30 193592 (M273) the maker already fitted the improved solenoid, under part number A272 051 01 77. The oil-leaking solenoid is a problem on engine numbers below 2729..30 895140 and 2739..30 236360.

The prevention is the oil stop cable, which has to be fitted between the solenoid and the harness: the two wires contain a sealed copper sleeve covered with heat-shrink tubing, and that stops the oil from travelling further. It is available from the factory and is not expensive: the two solenoids were around 33 euros and the two oil stop cables around 31 euros, while replacing the control unit is a four-figure sum.

The tensioner does not hold the chain, the chain goes slack and the valve timing shifts. The symptom is a metallic rattle audible at a cold start which quietens after a few seconds. If the chain jumps, the valves and the pistons meet.

The same noise is also produced by a failure of the camshaft adjuster, and there the fault code gives the direction: P001685, P001785, P001177 and P001885 belong to it.

The repair is not an engine strip. The tensioner can be reached after the charge air cooler and the front cover have been removed, and from there two screws hold it. The prevention is fitting the check valve into the oil supply.

The repair is typically replacing the mechatronics unit or the conductor plate together with an oil change: not the whole gearbox. The complaint is most common on the longitudinal platforms from around 2005 to 2015 (C/E/ML and the early GLC).

The 9G-Tronic (725.0) is a different design; this applies only to the 7G.

The rear air springs of the 2003 to 2009 E-Class (W211) are particularly prone to leaking along the clamps. If the system leaks, the compressor runs continuously, overheats and fails, so an unrepaired leak takes the compressor with it.

Symptoms: one corner of the car sinks by morning, an "Air Suspension" or level control fault message, and compressor noise while standing. It was an option (or standard) across several generations from the W220 S-Class onwards; the estate E-Class often has air springs with level control on the rear axle as standard.

On a damaged liner the cylinder wall becomes scored, and from then on the piston ring no longer seals: oil consumption rises and compression falls. The same material also wore the valve guides seriously. The maker finally switched to the Nanoslide coating, precisely because of Silitec's cylinder scoring problems.

It is not the mileage that marks out the affected cars but the engine number. Into engines above 2789xx 30 266191 the maker already fitted cast-in iron liners, which means that from there on this failure mode does not exist. It is reported on examples with an engine number below that, typically above 120 000 kilometres.

The symptom is blue smoke, rising oil consumption and uneven running. The repair is machining or replacing the block, which makes this the most expensive failure on this engine.