Faulty gear shift module

The module that reads a shift-by-wire selector and reports which gear the driver has asked for.

The context that makes its faults serious: on these vehicles there is no mechanical connection between lever and gearbox. Moving the selector is a request that a module interprets and an actuator carries out — so park is something the vehicle is doing on your behalf rather than a mechanical fact.

Its failures are usually one of two shapes. A position between detents — a reading corresponding to no valid gear — is rejected rather than guessed at, which is correct behaviour for a safety-critical input. Or the redundant sensors disagreeing: two or three watch the same lever precisely so no single failure can be believed, and one drifting makes the whole reading untrustworthy.

What follows is worth stating for the driver rather than the technician. The vehicle may refuse to leave park, refuse to select park, or — the dangerous one — display a gear it is not actually in, which is the reading somebody acts on. So apply the parking brake deliberately, and if the car rolls when the brake pedal is released it is not in park whatever the dashboard says.

The commonest physical cause is spilt liquid. The selector sits in the centre console where drinks land, and sticky residue bridging contacts produces exactly the disagreeing-sensor picture — and explains the version that comes and goes.