What it usually turns out to be
A0B4 is stored by the CAS against the start itself: it drove terminal 50, the starter command, and the engine did not start. Anything on that path can cause it — the starter or its solenoid, the heavy feed and ground the starter needs, a weak battery, or the terminal 50 wire between the CAS and the starter.
On early E-series cars with an electric steering lock, the CAS will not issue the start at all until the lock reports "unlocked and secured"; BMW lists that among the start conditions in SI B61 29 06. A steering-lock warning stored beside A0B4 changes where to look.
What gets replaced first, and why that is wrong
The CAS module and the starter both get quoted for this code. A voltage-drop test on the starter feed and ground while cranking, and a check that terminal 50 actually reaches the starter, tell them apart.
Generations: E90
Modules involved: CAS (Car Access System)
From a documented source: ISTA fault text as quoted on e90post.com and bimmerfest.com; BMW SI B61 29 06 (ELV).
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