After a collision the visible damage is only part of the job. The airbag module — MRS on the E90, E91 and E92, ACSM on the E60, E61, E63 and E64 from September 2005, the E85 from January 2006, the E70, E71 and E93, and on F- and G-series — stores crash data: deployment records and internal lock flags a scan tool cannot clear. Until that data is reset on the bench, or the module is replaced and coded, the airbag light stays on and the restraint system stays in fault.

Pyrotechnic devices are one-shot. Fired seat-belt pretensioners and the battery safety terminal — the small charge on the positive battery post that disconnects the battery in a crash — have to be replaced before the system goes back online.

Driver-assistance systems need their own attention. The windscreen camera (KAFAS) must be recalibrated after glass or bracket work, and the radar behind adaptive cruise needs re-alignment after front-end repair — 'camera calibration failed' and 'restricted function' messages are the usual signature. Calibration runs through the factory service functions and is confirmed on the road.

Diagnosis first: which devices fired, which sensors survived, what the module logged. BMW and Mini 2010+, Rolls-Royce 2007+ — including post-accident cars that need the safety system brought back properly.