P0556: Brake Booster Pressure Sensor Circuit Range/Performance
What to do
Check the pedal effort carefully. Press the brake several times with the engine off to exhaust the reserve, then start the engine — the pedal should sink slightly as vacuum builds. If it does not, assistance is genuinely lacking and the car needs attention before further driving.
What this code means
The brake booster vacuum reading is implausible — not falling when the brakes are applied, or failing to recover afterwards. This is the code in the group most likely to be reporting a genuine vacuum problem rather than a sensor fault, because a leaking booster diaphragm, a perished hose or a tiring vacuum pump all produce exactly this pattern.
A vacuum that recovers slowly after repeated pedal applications is the classic signature of a pump or leak, and is easy to observe on live data.
Likely causes
Start here
- Leaking brake booster diaphragm · specialist
- Faulty vacuum pump · moderate
- Disconnected or leaking vacuum hose · easy
- Faulty brake booster pressure sensor · moderate
Less common
- Failed vacuum check valve · easy
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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