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 maker's own bulletin gives two fault codes for this: 1200 and 1208. What is typical is that the code comes straight back after being erased, on restart, and that is what sets it apart from a one-off disturbance. The check engine light comes with it, along with uneven running and misfires, and the car can even stop.

For the repair the engine has to come out, but the cylinder heads do not have to come off. The counterweight has to be replaced along with the shaft, and the kit belongs to the displacement: A272 030 00 13 for the 2.5 litre, A272 030 01 13 for the 3.0 and A272 030 02 13 for the 3.5. In the same job the maker calls for the oil pump and the chain tensioner to be replaced, and for all four camshaft adjustment solenoids to be replaced with A272 051 01 77. The chain tensioner is single use: if it is taken out or readjusted after fitting, the chain is over-tightened, and that destroys the cylinder head. The repair is over 4000 dollars, because the labour with the engine removed is around 20 hours.

The complaints turned into a class action in the United States (Suddreth and Dunton versus Mercedes-Benz USA). The court finally dismissed the claim: the gears fail between 60 000 and 80 000 miles (about 95 000 to 130 000 km), that is outside the warranty period, and the maker was not held liable. According to the case material the maker knew about the fault, issued repair bulletins and finally switched to a stronger gear.

If the balance shaft repair is in the service book, the stronger gear is part of it.

The complaints turned into a class action in the United States (Suddreth and Dunton v Mercedes-Benz USA). The court finally dismissed the claim: the gears fail between 60,000 and 80,000 miles (roughly 95,000 to 130,000 km), so outside the warranty period, and the maker was not held liable. According to the case material the maker knew about the fault, issued repair bulletins, and eventually switched to a stronger gear.

If the service book shows a completed balance shaft repair, the stronger gear is fitted.

The case papers also name the M273 V8, but the M273 description does not cover this fault. This applies only to the M272.

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.

The symptoms are chain noise from the front of the engine and fault codes pointing at the camshaft. With slipped timing the engine is flat, uses more fuel and can also misfire.

With a broken linkage the flaps do not move, which means the intake manifold stays in one position. The symptom is a noticeable loss of power and a fault lamp coming on.

The cause is therefore not the age of the linkage but the oil: until the leak is stopped, things end up in the same place again after a replacement. Repair kits are available on the market, but they do not deal with the leak that causes it.