C05B8 code meaning
Brake Booster Motor A Position Sensor Supply Circuit/Open
What C05B8 means
The rotor position sensor of booster motor A – Hall, magnetoresistive or a digital SENT angle sensor – is fed from a regulated 5 V output of the brake control unit. C05B8 reports that this feed is interrupted. No manufacturer documents the code publicly; GM's 2019 integrated brake control lists supply faults of its two motor position sensors under C05D1 and C05BB only. Denso's sensor patent shows why an open feed is hard to tell from a dead sensor: with a pull-down input the signal simply falls to 0 V. Hitachi Astemo's steering patent therefore monitors each sensor supply separately so the failed one can be identified. Without a rotor angle the booster cannot commutate its motor, so assist is cut back or lost and the brake warning appears.
C05B8 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 C05B8
- Brake warning lamp and message such as service brake assist
- Noticeably harder brake pedal
- Regenerative or electronic brake functions unavailable
Common causes of C05B8
- Most likelyBroken supply wire or pin between control unit and motor position sensor
- PossibleSensor supply output in the brake control unit failed
- Less likelyInternal open in the position sensor's supply path
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 C05B8 means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| B8 | The exact fault in that area. |
What to do next
- Note C05B8 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 C05B8
What does C05B8 mean?
C05B8 means "Brake Booster Motor A Position Sensor Supply Circuit/Open". The rotor position sensor of booster motor A – Hall, magnetoresistive or a digital SENT angle sensor – is fed from a regulated 5 V output of the brake control unit. It is a generic code, so it means the same on every make.
Is it safe to drive with C05B8?
Drive gently and only to a workshop. With C05B8 the car can go into limp mode (reduced power) to protect itself.
Will C05B8 fail an emissions test?
C05B8 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 C05B8 to fix?
Difficulty: hard. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will C05B8 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.