C05B9 code meaning
Brake Booster Motor A Position Sensor Supply Circuit Low
What C05B9 means
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.
C05B9 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of C05B9
- Brake warning lamp, often with several sensor codes at once
- Brake pedal feels hard, assist reduced
Common causes of C05B9
- Most likelySupply line chafed through and shorted to ground
- PossibleDefective position sensor or another sensor on the same rail loading the supply
- Less likelyWeak sensor supply regulator in the control unit
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Booster motor a pos sensor supplyElectric brake booster
What each part of C05B9 means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| B9 | The exact fault in that area. |
What to do next
- Note C05B9 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about C05B9
What does C05B9 mean?
C05B9 means "Brake Booster Motor A Position Sensor Supply Circuit Low". C05B9 is set when the feed voltage for motor A's position sensor sags below its lower limit. It is a generic code, so it means the same on every make.
Is it safe to drive with C05B9?
Drive gently and only to a workshop. With C05B9 the car can go into limp mode (reduced power) to protect itself.
Will C05B9 fail an emissions test?
C05B9 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is C05B9 to fix?
Difficulty: hard. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will C05B9 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.