The symptom is a rattle on a cold start. The stretched chain makes the timing slip, and if the chain jumps, the valves and the pistons meet.

The pulse wheel is press-fitted onto the camshaft, and it shifts a little with every start. That shifting significantly speeds up the stretching of the chain.

The symptom is difficulty starting, which can grow as far as complete failure. The engine starts hard when cold, and as the valve timing drifts it loses power.

The repair is therefore not just the chain: the stretched chain and the faulty pulse wheel have to be replaced together. If only the chain is renewed, the cause stays in place and the next chain stretches the same way. So look for both of them on the invoice.

On engines built before 2014 the phase regulators also failed quickly and started to crack. The maker revised them, which pushed the fault later, but did not eliminate it.

On the affected cars the camshaft can break, and in doing so it damages the engine and shuts it off. The engine cannot then be restarted, the car comes to a halt, and in the words of the notice this increases the accident hazard.

As a precaution the maker prescribed replacement of the camshafts. The campaign started in November 2015, the target date for completion was the end of 2016, and the replacement is free for the owner.

At a purchase two things have to be cleared up: does the car fall into the production window, and if so, has the recall been carried out on it. The franchised dealer can answer that from the chassis number.

With a broken lobe the affected valve does not open, the engine runs unevenly, and the broken piece causes damage inside the engine. The manufacturer issued a recall and the replacement is free.

The characteristic course of events: the panoramic roof glass shatters while driving, with no impact, with a loud bang. Across the whole group 108 complaints use the wording "exploded / shattered into pieces", 155 say it happened while driving, 38 specifically note that nothing struck the roof, and 43 complaints mention injury or cuts from the glass.

By measurement the affected group is primarily the W205 C-Class (2015 to 2017), plus the MFA platform GLA (X156) and CLA (C117) in the 2014 to 2015 model years.

There is no rattle beforehand and no warning. The traces of a replacement show on the seal and the bonding. The replacement is not cheap, and the broken glass falls into the cabin.

The complaint database is American, and the panoramic roof is more common there as an option. The phenomenon itself does not depend on the market.

The symptom: the engine warning light comes on, and diagnostics returns a fault code complaining about crankcase ventilation valve performance. Alongside it you get a fluctuating idle, a whistling or sucking noise from the engine bay, and sometimes harder starting.

The valve is part of the intake system, and if it sticks open or closed the engine either draws false air or the pressure in the crankcase rises. The latter pushes the crankshaft seals out, and from then on you have an oil leak. The valve itself is a cheap part, but getting to it is not always cheap.

On the same engine generation the complaints also include 27 reports of a cracked piston (weighted to the C 300 in 2015 and 2017). That is much rarer. If the engine burns oil or knocks, do not think only of the ventilation valve.

The official data is American, and the 2.0 petrol is far more common there than in Europe.

The control unit logs it too: code P0128 means that the coolant temperature stays below the regulating temperature of the thermostat. The symptom is that the engine does not warm up properly even after a long time, the heater is lukewarm, consumption rises and the engine management light comes on as well.

An engine running cold for a long time wears faster, because cold oil does not lubricate the same way and fuel dilutes it in a greater proportion. The thermostat is built into the coolant circuit, so its replacement involves draining coolant.