DV6TED4 1.6 HDi

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.

A whine or rattle from the turbo, a loss of boost, blue smoke and eventually an oil-pressure or boost warning. The narrow oil feed pipe to the turbocharger cokes up when oil changes are stretched or the wrong grade is used. The bearing then starves, and debris from a failed turbo can travel through the engine afterwards.

Check: Read the service book before you drive the car and treat stretched or missing oil changes as a turbo living on borrowed time. Listen for a whine on boost and check the intake pipework for oil.

Sudden loss of power on a hard pull, limp mode that clears on restart, underboost or overboost codes. Soot binds the variable vanes or the actuator linkage seizes.

Check: Do a full-throttle pull in third from 1,500 rpm to the limiter. Hesitation or a drop into limp mode is this fault. Do this twice.

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.