On the 2.0 turbo four cylinder (M274, and the transverse M270) the control valve of the crankcase ventilation system fails early. The official database holds 259 reports on it, almost exclusively on the 2015 to 2019 C 300 and GLC 300 versions.

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.