C05B9Brake Booster Motor A Position Sensor Supply Circuit Low
- Generic OBD-II code · Chassis
- Can trigger limp mode
- How the car detects it
- C05B9 is set when the feed voltage for motor A's position sensor sags below its lower limit. No manufacturer publishes C05B9; as a reference point, GM's 2019 brake module calls its 5 V sensor supplies low below 4.85 V lasting 100 ms, but books that under C05D1 or C05BB. A low supply usually means the line is shorted to ground or a sensor is drawing too much current. Supplies of this kind are often tracking regulators such as TI's TPS7B4253-Q1, built for sensors on long cables: the output is short-proof to ground and current-limited at typically 450 mA, so instead of failing it lets the voltage collapse. Since the same rail can also feed pedal sensors, related codes may appear together. Motor commutation becomes unreliable and assist is reduced or removed.
- What you may notice
- Brake warning lamp, often with several sensor codes at once
- Brake pedal feels hard, assist reduced
- Likely causes
- LikelySupply line chafed through and shorted to ground
- PossibleDefective position sensor or another sensor on the same rail loading the supply
- RareWeak sensor supply regulator in the control unit
- Repair
- Hard · Usually a job for a workshop.Typical cost: €150 – €3,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.