C0556
Brake Booster Pressure Sensor Performance
The brake booster vacuum or pressure sensor signal does not respond plausibly to engine vacuum or electric booster activity.
- Check engine light: Not always
- Emissions related: No
- Repair difficulty: medium
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
Common causes
Ordered by how often each turns out to be the cause.
- Vacuum leak in booster lineCommon
- Vacuum pump weakPossible
- Sensor element aged beyond calibration windowLess likely
Parts involved
- Brake Booster Vacuum Sensor
- Vacuum Pump
C0556 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.