The fault concentrates on engines built between 2012 and early 2015; after February 2015 they got an improved guide. In the 3 Series the 320i and the 328i are affected.

You can hear the noise before the chain jumps.

It causes damage along two paths, and the less known one comes first. The broken pieces of plastic are sucked into the oil pickup tube, block the way of the oil there, and destroy the bearings before the chain has even jumped. The other path is the chain or the tensioner letting go: then the oil pressure disappears within seconds, and the engine can seize without stopping.

The warning is not the lamp. This fault does not always light a warning, so the noise is what to listen for: between 1500 and 2500 revs a high, whistling whine comes from the lower part of the engine, and at a cold start a rattle of one to five seconds is audible. The maker issued its own service bulletin for the whining noise on the N20 and the N26.

On the factory chain the fault is reported above 80 000 miles, that is above roughly 129 000 kilometres, but in the American settlement the maker took it on earlier as well: below 70 000 miles, that is 113 000 kilometres, the full cost, and above 100 000 miles or eight years nothing at all. The affected set is the N20 and N26 engines built between 2012 and 2015.

The repair costs between 4000 and 6000 dollars, because the work on the timing and on the oil pump go together. If someone has replaced the timing chain, the oil pump chain has to be replaced with it: if the job is left half done, the engine has to come apart again in a few years.

Three things can happen. The oil stretches the belt and it can jump off (on a single belt engine that means an instant loss of the water pump and the alternator); oil getting into the alternator destroys the windings; and oil reaching the hot exhaust smokes and gives a burnt smell, and the official complaints do include burning smells and smoke.

The leak is visible: five minutes on a lift settles it. The gasket itself is a cheap part; the labour, the ruined belt and the alternator take the money.

The first is wear of the clutch pack and the fluid: in a slow, tight corner (a car park, a roundabout) the car jerks and "hops", as if the drive grabbed and let go. You can also get a jerk when pulling away on a slope. The second is the plastic actuator gear: the position signal is lost, the brake, ABS and 4x4/DSC lights come on together on the dash, and a clicking is audible from the transfer case when you switch the ignition off.

Workshops often propose a full transfer case replacement straight away. But the plastic gear can be replaced on its own, and the wrong tyre size or a mixed set of tyres also imitates the jerking, because the transfer case then slips continuously. So clear up the tyres and the fault code first, before you pay for a transfer case replacement.

The first is the mechatronics unit: the valve body and the control electronics in one. If the solenoids or the pressure regulation weaken, the gearbox jerks, the D/R engagement is delayed, it logs a gear ratio fault code, and finally drops into limp mode (often staying in 2nd or 3rd).

The second, and often misdiagnosed, is the connector sleeve of the mechatronics unit: the O-ring of the plastic sleeve ages, and ATF leaks from the side of the housing, exactly where the wiring harness enters. This is easy to mistake for a rear crankshaft seal. The trail starts from the driver's side of the housing, not from the clutch side.

On cars built in 2023 there was also a separate safety recall for a welding fault in the mechatronics unit. That is not the same as the wear related sleeve and the valves.

Check the fluid level, the area around the sleeve and the fault codes first; do not accept a "full gearbox replacement" quote straight away.