A loose, corroded, or partially broken electrical connector at the brake booster pressure sensor.; Chafed or intermittently shorted wiring in the sensor harness, often where it routes near moving components or heat sources.; A failing brake booster pressure sensor that drops its signal under vibration or temperature changes.; A poor ground connection shared by the sensor circuit, causing voltage fluctuations that mimic a sensor fault.; A brake booster with an internal vacuum leak that causes pressure readings to swing outside the expected range.; A failing powertrain control module, though this is rare and should only be suspected after all circuit tests pass.

DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P0559

P0559Brake Booster Pressure Sensor Circuit Intermittent

Q: What does code P0559 mean?

Brake Booster Pressure Sensor Circuit Intermittent. P0559 indicates that the powertrain control module has detected an intermittent fault in the electrical circuit of the brake booster pressure sensor, which monitors the vacuum or hydraulic assist available for braking. The circuit works correctly at times but drops out or returns erratic readings, so the module cannot trust the sensor signal continuously.

P0559 indicates that the powertrain control module has detected an intermittent fault in the electrical circuit of the brake booster pressure sensor, which monitors the vacuum or hydraulic assist available for braking. The circuit works correctly at times but drops out or returns erratic readings, so the module cannot trust the sensor signal continuously.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

Illumination of the brake system warning light or the check engine light on the dashboard.
A brake pedal that occasionally feels firmer or harder to push than usual, especially during repeated stops.
Intermittent reduction in power brake assist, often noticed as inconsistent pedal effort from one stop to the next.
A stored trouble code with no noticeable change in braking performance, since the fault is intermittent.
Possible activation of a related warning message such as 'Brake Assist Unavailable' on vehicles with advanced braking features.
Occasional erratic operation of cruise control or stability control systems that rely on brake pressure data.

Common causes, most likely first

A loose, corroded, or partially broken electrical connector at the brake booster pressure sensor.
Chafed or intermittently shorted wiring in the sensor harness, often where it routes near moving components or heat sources.
A failing brake booster pressure sensor that drops its signal under vibration or temperature changes.
A poor ground connection shared by the sensor circuit, causing voltage fluctuations that mimic a sensor fault.
A brake booster with an internal vacuum leak that causes pressure readings to swing outside the expected range.
A failing powertrain control module, though this is rare and should only be suspected after all circuit tests pass.

How to diagnose and fix it

Inspect the brake booster pressure sensor connector and harness for corrosion, loose pins, or chafed insulation, and repair as needed.
Clear the code and perform a wiggle test on the sensor wiring while monitoring live sensor data to reproduce the intermittent fault.
Check the sensor reference voltage and ground at the connector with a multimeter, comparing readings to the manufacturer's specification.
Test the brake booster pressure sensor output with a scan tool or oscilloscope, looking for dropouts or erratic voltage swings during brake pedal application.
Verify that the brake booster holds vacuum properly and that no check valve or hose leak is causing abnormal pressure readings.
Replace the brake booster pressure sensor if circuit and booster tests confirm the sensor itself is dropping its signal.
After repair, clear the codes and road test the vehicle through several stop-and-go cycles to confirm the code does not return.

Intermittent faults that fool good technicians

An intermittent code like P0559 often sets only once during a specific driving condition, such as a hard stop or a cold start, so the fault may not be present when the vehicle is in the bay.
A chafed wire that only shorts when the engine moves under torque can pass every static multimeter test, making a live-data wiggle test far more valuable than a simple resistance check.
Technicians sometimes replace the brake booster pressure sensor only to find the real problem is a corroded ground shared with other sensors, which corrupts the reference voltage.
A vacuum leak at the brake booster check valve can cause pressure readings to swing enough to set P0559 without any visible damage to the sensor or its wiring.
Vehicles with electronic stability control may store P0559 alongside codes for the yaw rate or wheel speed sensors, leading some to misdiagnose the fault as a stability system failure rather than a brake booster circuit issue.

Can I keep driving with P0559?

The vehicle remains driveable because the fault is intermittent, but you should avoid heavy braking or towing until it is repaired, since a sudden loss of power assist can increase stopping distance. If the brake pedal feels consistently hard or the warning light stays on, stop driving and have the system inspected immediately.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.