Failed brake booster pressure sensor; Damaged, disconnected, or corroded wiring or connector at the sensor; Leaking or disconnected vacuum hose between the engine and brake booster; Faulty brake booster check valve; Internal leak in the brake booster diaphragm; Corroded or damaged PCM connector pins or faulty PCM (rare)

DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P0555

P0555Brake Booster Pressure Sensor Circuit

Q: What does code P0555 mean?

Brake Booster Pressure Sensor Circuit. P0555 indicates that the powertrain control module (PCM) has detected an abnormal signal from the brake booster pressure sensor circuit, which monitors the vacuum or hydraulic pressure assist provided to the brake pedal. The computer compares the sensor's voltage to expected values and sets this code when the signal falls outside the normal operating range for a calibrated period.

P0555 indicates that the powertrain control module (PCM) has detected an abnormal signal from the brake booster pressure sensor circuit, which monitors the vacuum or hydraulic pressure assist provided to the brake pedal. The computer compares the sensor's voltage to expected values and sets this code when the signal falls outside the normal operating range for a calibrated period.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

Illumination of the brake warning light and/or check engine light on the dashboard
A noticeably harder brake pedal that requires more effort to stop the vehicle
Inconsistent or poor brake performance, especially during repeated stops
A hissing noise from the brake booster area when the pedal is pressed
Often no symptoms besides the warning light, with normal brake feel
Possible activation of a limp-home mode or reduced power assist in some vehicles

Common causes, most likely first

Failed brake booster pressure sensor
Damaged, disconnected, or corroded wiring or connector at the sensor
Leaking or disconnected vacuum hose between the engine and brake booster
Faulty brake booster check valve
Internal leak in the brake booster diaphragm
Corroded or damaged PCM connector pins or faulty PCM (rare)

How to diagnose and fix it

Inspect the sensor connector and wiring for looseness, corrosion, or damage and repair as needed
Check all vacuum hoses and connections between the engine and brake booster for leaks or disconnection
Test the brake booster check valve for proper operation and replace if faulty
Measure the sensor reference voltage and signal voltage with a multimeter and compare to manufacturer specifications
Perform a vacuum test on the brake booster to confirm it holds vacuum and replace if defective
Replace the brake booster pressure sensor if testing confirms it is faulty
Clear the diagnostic trouble codes and verify the repair by test-driving the vehicle

Don't overlook the simple vacuum leak

A cracked or disconnected vacuum hose at the brake booster is a frequent cause of P0555 that is often missed because the hose can look intact from the outside.
The code can be set by a failing brake booster check valve that allows vacuum to bleed off overnight, causing a hard pedal on first start.
Some vehicles use a hydraulic pressure sensor instead of a vacuum sensor, so confirm the system type before ordering parts.
A weak engine vacuum due to a separate engine performance issue can trigger P0555 even when the booster and sensor are healthy.
The PCM may not illuminate the brake warning light immediately; the code can be stored pending without any driver complaint.

Can I keep driving with P0555?

If the brake pedal feels normal and only the warning light is on, you can drive cautiously to a repair shop, but avoid heavy braking or towing. If the pedal is hard or braking performance is reduced, stop driving and have the vehicle towed to prevent an accident.

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.