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P codepowertrainHigh severity

P0556Brake Booster Pressure Sensor Circuit Range/Performance

The brake booster pressure sensor signal is electrically valid but does not match the pressure or vacuum the PCM expects, indicating a rationality fault.

Definition

The PCM compares the brake booster pressure sensor reading against expected values — near barometric with the key on and engine off, and a steady vacuum with the engine running — and against changes expected during brake applications. P0556 sets when the signal stays within electrical limits but is stuck, slow, or inconsistent with those conditions. The MIL illuminates, and because this monitor often protects power brake assist on start-stop, hybrid, and turbocharged vehicles, an undetected vacuum loss can result in a hard brake pedal and increased stopping distance.

Common causes

  • Leaking, kinked, or disconnected booster vacuum hose
  • Skewed or lazy booster pressure sensor
  • Failed booster check valve bleeding down vacuum
  • Internal brake booster diaphragm leak
  • Auxiliary vacuum pump not producing rated vacuum
  • Partially corroded connector adding resistance to the signal

Common misdiagnoses

  • Brake booster replaced when a cheap hose or check valve is leaking
  • Sensor replaced when the actual vacuum supply is low
  • Master cylinder replaced for hard-pedal complaints
  • Vacuum pump replaced when its supply hose leaks

Troubleshooting steps

  1. 1.Compare the sensor PID to a gauge

    Tee a vacuum gauge into the booster supply; engine idling, the gauge and the scan-tool PID should agree within about 1-2 inHg.

  2. 2.Watch the PID during brake applications

    Each firm brake apply with the engine idling should consume vacuum and show a momentary reading change that then recovers; a frozen PID indicates a stuck sensor.

  3. 3.Test the check valve and hose

    The booster should hold vacuum for several minutes after engine shutdown; a rapid bleed-down means a leaking valve, hose, or booster diaphragm.

  4. 4.Listen for booster diaphragm leaks

    With the engine idling, press the brake pedal and listen for a steady hiss inside the cabin under the dash, which indicates an internal booster leak.

  5. 5.Verify the auxiliary vacuum pump if equipped

    Command the pump with a scan tool and confirm it pulls the booster to specification (typically 15+ inHg) within a few seconds.

  6. 6.Replace the sensor only after supply checks pass

    If actual vacuum is correct but the PID disagrees or lags, replace the booster pressure sensor and re-verify.

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