P0556 Brake Booster Pressure Sensor Circuit Range/Performance

P0556 points to a range or performance issue in the brake booster pressure sensor circuit. The signal exists but does not match expected booster vacuum or pressure behavior.

Code details

Generic OBD-II · P0

System: Speed / Idle
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace the sensor before checking booster vacuum supply, check valve behavior, and actual pressure response.

Symptoms of P0556

Unstable, high, or low idle, or stalling.
Erratic speedometer, cruise-control, or shift behaviour.
Check-engine light with a hesitation or surge.

Common causes of P0556

A dirty throttle body, idle-control valve, or throttle-position sensor.
A failing vehicle-speed or wheel-speed sensor, or its wiring.
Vacuum leaks or a related sensor forcing the idle/speed strategy.

Useful checks before replacing parts

Compare booster pressure data with brake pedal application, manifold vacuum, vacuum pump command, engine load, and freeze-frame conditions.
Inspect hose leaks, check valve function, sensor response speed, connector condition, wiring resistance, booster vacuum reserve, and pump output where equipped.
Verify sensor readings during repeated brake applications and engine-off vacuum reserve tests.

Related OBD-II codes

These nearby speed / idle entries can help compare wording and pin down the fault area.