CAS (Car Access System)

The CAS is the car's ignition switch and terminal controller. BMW's E90 training lists its jobs: switching the terminals and the KL15 and KL30g relays, validating the key and running the electronic immobilizer, acting as master for central locking, comfort access, the windows and the sunroof, controlling the electric steering lock where one is fitted, and keeping a redundant copy of the vehicle order and the service (CBS) data.

Generations follow the chassis: CAS2, CAS3 and CAS3+ through the E-series, CAS4 on the F01/F10/F25 family. On the F30 generation the job moved into the FEM, and from the F15 X5 and the G-series into the BDC.

Failures show as intermittent no-crank, "key not recognized", or a car that wakes but will not switch on terminal 15. On early E-series cars with an electric steering lock, the CAS will not allow a start until the lock reports "unlocked and secured", so a lock fault can read as a CAS fault. The common wrong turn is new keys or a new starter before the start chain has been checked link by link.