OBD-II DTC · SAE J2012
C05B9
Brake Booster Motor A Position Sensor Supply Circuit Low
What C05B9 means
C05B9 is a standard SAE J2012 powertrain diagnostic trouble code with the defined meaning: Brake Booster Motor A Position Sensor Supply Circuit Low. This is the OEM-agnostic definition; specific manufacturer interpretation may add detail (subsystem, bank, sensor location) but the core meaning is consistent across all OBD-II compliant vehicles since 1996.
Common symptoms when C05B9 is active
Overboost cut, underboost limp, wastegate stuck open / closed, swirl flap warnings, intermittent power-loss with no obvious mechanical fault.
Can this code be suppressed via ECU tuning?
Generally NOT recommended to suppress without fixing the underlying issue. Boost-related DTCs often indicate genuine turbo or wastegate problems that warrant inspection.
How to remove C05B9 with Softechpro V5
Recommended service: DTC OFF (boost branch). Step-by-step workflow:
- Diagnose the underlying boost issue first — boost-related codes usually indicate a real fault (wastegate, hose, MAP sensor).
- If the fault is benign or already fixed: open Softechpro V5 and load the bin.
- Pick DTC OFF and select the boost / turbo DTC group — Softechpro lets you suppress only the codes you specify, not the entire DTC table.
- Save the patched .bin and flash back. If the wastegate is actually stuck, suppressing the code will not fix the symptom.
Softechpro V5 supports 1,400+ ECU firmwares across Bosch (EDC15, EDC16, EDC17, MD1, MG1, ME7, MED17), Siemens / Continental (SID, SIMOS, EMS), Delphi (DCM, DDC), and Magneti Marelli platforms.