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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P0556

P0556Brake Booster Pressure Sensor Circuit Range/Performance

P0556 indicates that the powertrain control module (PCM) has detected an out-of-range or erratic signal from the brake booster pressure sensor, which monitors the vacuum or hydraulic pressure assisting your brakes. The PCM compares this signal to expected values under specific operating conditions and sets the code when the voltage or pressure reading falls outside the acceptable range or fails a rationality check.

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 or check engine light, often with no other noticeable symptoms
  • A brake pedal that feels unusually hard or requires more effort to depress
  • Inconsistent brake pedal feel, sometimes feeling normal and other times stiff
  • Reduced power brake assist, especially during low-speed maneuvers or emergency stops
  • In rare cases, a stored code without any drivability or braking complaints

Common causes, most likely first

  1. Faulty brake booster pressure sensor sending an erratic or out-of-range signal
  2. Damaged, corroded, or disconnected wiring or connector at the sensor
  3. Vacuum leak in the brake booster hose, check valve, or booster itself affecting pressure readings
  4. Failed brake booster check valve allowing pressure to bleed off
  5. Contaminated or degraded sensor due to oil or moisture intrusion
  6. PCM software issue or internal fault in the pressure sensor circuit

How to diagnose and fix it

  1. Connect a scan tool and record freeze-frame data to see the sensor voltage and pressure readings when the code set
  2. Visually inspect the sensor, connector, and wiring for damage, corrosion, or loose pins
  3. Check the brake booster vacuum hose and check valve for leaks or cracks using a vacuum gauge or smoke test
  4. Measure the sensor reference voltage and signal voltage with a multimeter and compare to manufacturer specifications
  5. Test the sensor with a vacuum pump or pressure gauge to verify it responds correctly to changing pressure
  6. Replace the brake booster pressure sensor if it fails electrical or pressure testing
  7. Clear the code and perform a road test to confirm the repair and ensure the code does not return

What a brake booster pressure code really means

  • A vacuum leak at the booster check valve or hose can mimic a failed sensor, so always verify vacuum integrity before replacing parts
  • The sensor may only act up when the engine is cold or under specific vacuum conditions, so a simple key-on test can miss the fault
  • Some vehicles use the brake booster pressure sensor for cruise control and hill-start assist, so a fault can disable those features without obvious brake symptoms
  • Corroded pins in the sensor connector are a common cause that a visual inspection alone may not reveal; a wiggle test with a scan tool connected can help
  • After replacing the sensor, some vehicles require a relearn or calibration procedure with a scan tool to avoid a repeat code

Can I keep driving with P0556?

You can generally drive the vehicle to a repair shop, but be aware that power brake assist may be reduced, increasing stopping distance. Avoid heavy braking or towing until the fault is repaired, and if the pedal feels hard or braking performance is noticeably degraded, stop driving and have the vehicle towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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

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