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 symptom shows up on a cold start first: a short, metallic rattle or knock from the front of the engine that quietens after a few seconds. Later an engine warning light and timing control fault codes join it.

In the official complaint database the four cylinder M271 model group (C 180/200/230/250, E 200, SLK 200/250, GLK 250, CLS 250) has 16 reports between 2008 and 2015, and 8 of those carry a mileage. The raw numbers fall between 61,000 and 152,000 km, from eight reports.

All 16 reports fall on the 2012 to 2014 model years of the C 250. The complaint texts repeatedly mention the maker's extended warranty.

On a warm engine the rattle cannot be heard.

After the recall is closed, the SBC pump unit still has a service life counter, the replacement is expensive, and the backup mode is noticeable on a test drive. On the post 2006 facelift W211 this system is gone.

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 early cars (1995 to 2001) had planetary gear and freewheel wear; after 1998 to 2001 the design improved, and on the used market the W203/W211/W220 era is the typical find.

• Up to 2003, engine management disturbances and leaking fuel lines on the diesels.
• Up to 2005, frequent battery drain (the onboard network manager software was updated in April 2005).
• Up to 2006, undersized ball joints and poor bushings, leading to heavy front and rear suspension wear.
• The flex discs (propshaft to gearbox and rear differential) are prone as well.

According to J. D. Power's measurement at the time, three year old W211 cars had 283 faults per 100 cars (industry average: 237).