What it does
BMW's component description is direct: the FEM provides the functions of the former footwell module (FRM), Car Access System (CAS), junction box electronics (JBE) and central gateway module (ZGM), and acts as the gateway for the other control units. It arrived with the F20/F30 family — the 1, 2, 3 and 4 Series from 2011 — while the F01/F10/F25 cars kept a separate CAS4 and FRM3, and the F15 X5 went straight to the BDC.
How it fails, and what gets replaced first by mistake
Because it is also the immobilizer and the gateway, a FEM fault can show as a no-start, a key that is not recognized or modules dropping off the bus — on top of the lighting, window and locking faults the FRM used to own. It holds the car's key and immobilizer data and is coded to the car, so a used FEM does nothing until that data is transferred and the module is programmed in.
Generations this applies to
Jobs where this was the fault
From a documented source: BMW TIS component description A173 (FEM); transpondery.com CAS/FEM/BDC list.
Is this module acting up on your car?
I come to the car, read the modules and trace the fault to the actual wire, connector or module.
Run a repair or body shop? BMW programming support for shops