What code 120308 means
B58-specific charging-pressure code. Mirrors 30FF but uses the B58 DME fault ID convention rather than the older hex-digit convention. Same diagnostic path: smoke-test, check N75, verify wastegate actuation live.
How to read a boost code
Boost codes mean the turbocharger is not delivering the pressure the DME asked for (underboost) or is delivering more than asked (overboost). Underboost is the common case on road-going BMWs.
- Smoke-test the charge pipes.
The most common cause of underboost on a turbo BMW is a cracked charge pipe or a failed seal at the throttle body or the intercooler.
- Test the N75 / wastegate regulation valve.
The N75 valve (or its modern equivalent) is a small electronic valve that regulates boost by controlling vacuum to the wastegate. A failed N75 produces overboost or underboost depending on which way it stuck.
- Check wastegate actuation live.
Command the wastegate closed and verify it holds boost. If it does not, the actuator diaphragm is likely torn.
- Inspect the turbo for shaft play.
On high-mileage turbos, excessive shaft play allows compressor bypass that looks like underboost.
Related codes in the same family
30FF — Turbocharger charge-air pressure too low (underboost).
A DTC tells you what a control module decided is outside its expected range. It does not automatically identify a failed component. Read freeze frames, compare related codes, and inspect live data before changing parts. Service functions and coding operations are write paths — use a battery maintainer where the procedure requires stable voltage and never perform writes you do not understand.
120308 questions
What does code 120308 mean?
Charging pressure control: too low (B58).
Is 120308 a BMW-specific code?
Yes — {code} is a BMW-specific fault code, not a generic OBD-II code.
How do I read 120308 with Beemuu?
Connect on K+DCAN or ENET/DoIP, run a vehicle scan, and look up 120308 in the fault list. Beemuu preserves the code, description and freeze frame before you clear anything.
What does the severity mean?
Critical means the code is usually associated with drivability or safety; warn means investigate soon; info means it is a state or condition rather than an outright fault.