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DTC codes / C05xx · Brakes, steering and suspension / C0556

C0556Brake Booster Pressure Sensor Performance

C0556 indicates that the anti-lock brake system (ABS) module has detected a performance problem with the brake booster pressure sensor, which monitors the vacuum or hydraulic pressure assist provided to the brake pedal. The computer compares the sensor signal against expected values under specific operating conditions and sets this code when the signal is out of range, erratic, or not responding as expected.

SystemChassis
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaBrakes, steering and suspension

Symptoms you may notice

  • Illumination of the ABS warning light and possibly the traction control and stability control lights
  • A hard or stiff brake pedal that requires extra effort to stop the vehicle
  • Reduced brake assist, making the pedal feel less responsive than usual
  • Often no symptoms besides the warning light, especially if the sensor fault is intermittent
  • In some cases, the ABS system may disable itself, but normal braking usually remains functional

Common causes, most likely first

  1. Faulty brake booster pressure sensor
  2. Damaged, corroded, or loose wiring or connector at the sensor
  3. Vacuum leak in the brake booster hose or booster itself, affecting pressure readings
  4. Failed brake booster (internal leak or diaphragm damage)
  5. Contaminated or degraded brake fluid affecting hydraulic pressure sensor readings
  6. Faulty ABS control module (less common)
  7. Engine vacuum supply issues (e.g., vacuum pump failure on engines that use one)

How to diagnose and fix it

  1. Perform a visual inspection of the brake booster pressure sensor, its connector, and wiring for damage or corrosion
  2. Check for technical service bulletins (TSBs) and scan all modules for related codes to understand the full context
  3. Use a scan tool to view live data from the brake booster pressure sensor and compare it to expected values under different pedal positions
  4. Test the sensor with a multimeter according to the manufacturer's specifications (e.g., voltage or resistance changes with pressure)
  5. Inspect the brake booster vacuum hose and booster for leaks using a vacuum gauge or smoke machine
  6. If the sensor tests bad, replace it and clear the codes; if wiring is faulty, repair the harness
  7. After repairs, clear codes and perform a road test to verify the fix and ensure the code does not return

Don't overlook vacuum leaks or wiring

  • A vacuum leak at the brake booster or its hose can mimic a faulty sensor, causing the code to set even when the sensor is good.
  • Corroded or chafed wiring at the sensor connector is a common cause that is often missed during a quick visual inspection.
  • The brake booster pressure sensor is sometimes integrated into the ABS module or booster assembly, making replacement more involved on certain vehicles.
  • This code can be set by a low brake fluid level or air in the hydraulic system, so always check the basics before replacing parts.
  • Intermittent sensor failures may not set the code during a short test drive; extended live data monitoring is often needed to catch the glitch.

Can I keep driving with C0556?

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

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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