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BMW fault code 120308: boost pressure too low / too high
Boost (charging) pressure control fault. What the code means, its most common causes and what to do.
Codes in this family
Six-digit BMW codes stored by the engine control unit (DME). The same code appears in this form in ISTA and in the MOSTUNED app.
| Code | Meaning |
|---|---|
| 120308 | Charging pressure control: Pressure too low |
| 120208 | Charging pressure control: Pressure too high |
| 120408 | Charging pressure control: Switch-off as consequence |
What does 120308 mean?
The engine control unit (DME) sets a target boost pressure at every moment and compares it with the measured value. If measured pressure stays clearly below target it stores 120308; if it overshoots, 120208. If the deviation persists the system switches boost control off and adds 120408, and the car limits power.
Most common causes
Top to bottom, starting with what is easiest to check.
| Cause | What to check |
|---|---|
| Charge-air leak | Check the hoses, clamps and plastic charge pipe between turbo, intercooler and throttle. This is the most common cause. |
| Wastegate actuator | The electric wastegate linkage may have play or have lost its adaptation. |
| Intake restriction | A dirty air filter or a collapsed intake hose makes target pressure hard to reach. |
| Pressure sensor | If the boost or manifold pressure sensor reads wrong, the DME sees a false deviation. |
| Exhaust side | A blocked catalytic converter or an exhaust leak reduces turbine efficiency. |
Can you drive with this code?
The car usually keeps driving at reduced power. Avoid high load and have the cause found; a turbo running against a leak works harder than it should to reach target.
How it relates to tuning
Raising power raises target boost; a tired hose or loose clamp that held factory pressure shows up here first. That is why we check the charge-air path before tuning. With the MOSTUNED Logger you plot target and actual boost on the same graph and see a leak while driving.
Read the code yourself, free
- 1 Install MOSTUNED on your iPhone.
- 2 Connect to the car with an ENET cable or WiFi-ENET adapter.
- 3 Open “Fault Codes” under Diagnostics; stored codes are listed with their descriptions.
- 4 Note the code, fix the cause, then clear it from the same screen and read again to confirm.
Frequently asked questions
What does BMW fault code 120308 mean?
The engine control unit (DME) sets a target boost pressure at every moment and compares it with the measured value. If measured pressure stays clearly below target it stores 120308; if it overshoots, 120208. If the deviation persists the system switches boost control off and adds 120408, and the car limits power.
Can I drive with code 120308?
The car usually keeps driving at reduced power. Avoid high load and have the cause found; a turbo running against a leak works harder than it should to reach target.
Does clearing 120308 fix the problem?
Clearing removes the entry, not the cause. If the cause remains, the code returns when the conditions occur again. The right order is: read the code, fix the cause, clear it and read again to confirm.
How do I read code 120308?
You read it for free with the MOSTUNED app: connect your iPhone to the car with an ENET cable or WiFi-ENET adapter and open Fault Codes under Diagnostics. Bluetooth OBD (ELM327) adapters do not provide this connection.
Does tuning cause this code?
Raising power raises target boost; a tired hose or loose clamp that held factory pressure shows up here first. That is why we check the charge-air path before tuning. With the MOSTUNED Logger you plot target and actual boost on the same graph and see a leak while driving.
I cannot find the cause. What should I do?
Send us your car model, the code and a Logger recording if you have one on WhatsApp and we will look at it together. Related codes: 120208, 120408.
Next step
- All BMW fault codes — B48 / G-series code guide
- Download the app — Fault reading and clearing are free
- ENET cable and adapter guide — What you need to connect
- Using the Logger — Narrow down the cause with live data
Contact
Not sure?
Send your model, hardware and goal — we will pick the right package together. We do not sell what does not fit.