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P codepowertrainHigh severity

P0557Brake Booster Pressure Sensor Circuit Low Input

The brake booster pressure sensor signal voltage is stuck below its normal operating range, usually from a short to ground or a failed sensor.

Definition

The PCM supplies the brake booster pressure sensor with a 5-volt reference and monitors the returned signal, which should stay within a defined window across all pressure conditions. P0557 sets when the signal drops below the low threshold (commonly under about 0.2V), indicating a short to ground, an open reference feed, or a failed sensor. The MIL illuminates and the PCM loses its ability to detect low brake booster vacuum, so on vehicles that rely on the sensor to activate an auxiliary vacuum pump, brake assist can degrade and the pedal may become hard, particularly during start-stop events.

Common causes

  • Signal wire shorted to ground in the harness
  • Failed brake booster pressure sensor
  • Open or shorted 5V reference circuit to the sensor
  • Water intrusion corroding the sensor or connector
  • Chafed harness near the booster or cowl
  • Faulty PCM input circuit (rare)

Common misdiagnoses

  • Brake booster replaced for an electrical circuit fault
  • Auxiliary vacuum pump replaced when the sensor circuit disabled it
  • Master cylinder replaced for pedal complaints
  • Battery/charging system suspected for a low-voltage signal

Troubleshooting steps

  1. 1.Confirm the low signal on the scan tool

    The booster pressure sensor voltage PID pinned near 0V regardless of engine state confirms the circuit fault.

  2. 2.Unplug the sensor and watch the PID

    If the voltage rises toward 5V with the connector off (on pull-up designs) the sensor is shorted internally; if it stays at 0V, the harness or PCM side is at fault.

  3. 3.Check the 5V reference at the connector

    With key on, verify 4.9-5.1V on the reference terminal; a missing reference can also drag the signal low and may set codes on other sensors sharing the circuit.

  4. 4.Test the signal wire for a short to ground

    With the sensor and PCM connectors unplugged, resistance from the signal wire to chassis ground should read open; near 0 ohms confirms a harness short.

  5. 5.Repair and verify sensor response

    After repair, apply vacuum to the sensor with a hand pump and confirm the signal voltage sweeps smoothly through its range.

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