N22B 2.2 i-DTEC

Rising oil level with a diesel smell, rough running, and at worst a knock and loss of compression. Repeated incomplete particulate-filter regenerations wash unburnt fuel past the rings and dilute the oil. On short-trip cars this eventually damages a piston. Cars used for longer journeys largely escape it, so how the car was driven matters more than the mileage.

Check: Check the oil level and smell it for diesel, ask directly what journeys the car did, and read the service record for short intervals or repeated top-ups.

Warning light, limp mode, oil level rising above the maximum mark from fuel dilution. The filter needs sustained hot running to regenerate. A car used for short urban trips never completes a regeneration cycle.

Check: Read the DPF soot load and the distance since the last regeneration on a scan tool, and check the engine oil level and smell it for diesel. Ask what the car was used for. A short-trip city diesel is the wrong car regardless of mileage.

Coolant loss with no visible external leak, white smoke, sweet smell. The EGR cooler cracks internally and leaks coolant into the intake. BMW recalled affected diesels over a fire risk from glycol deposits igniting.

Check: Check coolant level and look for unexplained top-ups in the service history. Verify the recall was carried out. It's free and searchable by VIN.