The heart of the four wheel drive in the longitudinal engine BMW X models (X3, X5 and the X1 E84) is the ATC transfer case. It has two faults, and you can feel both while driving.

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.