FixItFor.Us
P codepowertrainHigh severity

P0555Brake Booster Pressure Sensor Circuit

A general electrical fault has been detected in the brake booster pressure (vacuum) sensor circuit.

Definition

The PCM monitors a pressure sensor mounted on or plumbed to the brake booster to verify that adequate vacuum (or hydraulic pressure on some systems) is available for power brake assist, and on many vehicles to trigger an auxiliary vacuum pump or adjust throttle/idle strategy. P0555 sets when the sensor signal shows an open, short, or otherwise implausible electrical condition. The MIL typically illuminates and a brake warning may appear; the practical risk is that the PCM cannot detect low booster vacuum, so brake assist could degrade without warning, especially on turbocharged, hybrid, or start-stop vehicles that depend on the sensor to run a vacuum pump.

Common causes

  • Failed brake booster pressure/vacuum sensor
  • Damaged wiring or connector at the sensor (opens, shorts, corrosion)
  • Loss of 5V reference or sensor ground to the sensor
  • Vacuum hose leak or disconnection confusing plausibility checks
  • Water intrusion into the sensor from its cowl-area mounting
  • Faulty PCM input circuit (rare)

Common misdiagnoses

  • Brake booster replaced when only the sensor or wiring has failed
  • Master cylinder replaced for assist complaints
  • Auxiliary vacuum pump replaced when the sensor circuit prevents its activation
  • Vacuum check valve replaced without testing the circuit

Troubleshooting steps

  1. 1.Check the sensor PID against actual conditions

    Key-on engine-off the sensor should read near barometric pressure; with the engine idling it should show strong vacuum (roughly 15-20 inHg below atmospheric), and a frozen or pegged value points to the circuit.

  2. 2.Inspect the sensor connector and wiring

    Check for corroded terminals, broken wires at the connector, and water intrusion where the sensor mounts on the booster.

  3. 3.Verify reference voltage and ground

    Back-probe the connector and confirm 5V reference and a good ground (under 0.1V drop) with the key on.

  4. 4.Test the signal wire

    Confirm the signal circuit is not open or shorted between the sensor and PCM, and that signal voltage changes when vacuum is applied to the sensor with a hand pump.

  5. 5.Confirm booster vacuum supply

    Verify the hose from the intake (or pump) to the booster holds vacuum and the check valve works, since a mechanical vacuum problem can accompany the electrical code.

  6. 6.Clear the code and verify brake assist

    After repair, confirm normal pedal feel with the engine running and that the sensor PID responds to repeated brake applications.

Related codes