P0559Brake Booster Pressure Sensor Circuit Intermittent
The brake booster pressure sensor signal is dropping in and out intermittently rather than failing outright.
Definition
The PCM sets P0559 when the brake booster pressure sensor signal shows erratic behavior — sudden spikes, dropouts, or discontinuities that are electrically implausible — but returns to normal in between, pointing to an intermittent connection rather than a hard failure. The MIL may illuminate intermittently and the code may be stored as pending or history. The risk is the same as the hard-fault codes: during a dropout the PCM cannot verify brake booster vacuum, so an auxiliary vacuum pump may cycle erratically or a real assist loss could be missed, and intermittent faults left unrepaired usually progress to permanent ones.
Common causes
- Loose or corroded terminal in the sensor connector
- Chafed wiring that shorts or opens with vibration and engine movement
- Failing sensor with an intermittent internal element
- Water intrusion causing intermittent conduction in the connector
- Broken wire strands inside intact insulation at a flex point
- Poor chassis ground splice shared with the sensor
Common misdiagnoses
- Sensor replaced when a connector terminal has lost tension
- Brake booster replaced for an electrical intermittent
- PCM replaced for a wiring fault
- Vacuum pump replaced for erratic cycling caused by the signal
Troubleshooting steps
1.Review freeze frame and code history
Note the conditions (temperature, engine state, mileage since last fault) to reproduce the intermittent during testing.
2.Wiggle-test while recording the PID
Graph the sensor voltage on a scan tool while flexing the harness and tapping the connector and sensor; any glitch during manipulation pinpoints the fault location.
3.Inspect terminal tension
Remove the connector and check that each female terminal grips a test pin firmly; re-pin any loose cavities rather than just cleaning them.
4.Check for moisture and corrosion
Look for green residue or water in the connector, common near cowl drains, and repair the seal after cleaning.
5.Road test over rough surfaces while logging
Record the sensor PID during a drive over bumps; a clean, continuous trace after repair confirms the fix.