What causes fault code P0556?

• Most likely: Vacuum leak in hoses or check valve leading to the brake booster • Likely: Defective brake booster pressure sensor • Possible: Wiring issues (corrosion or short circuit in connector) • Rare: Faulty electric vacuum pump (if equipped) • Rare: Internal brake booster failure (diaphragm leak)

How do you fix fault code P0556?

STEP 1: Visually inspect all vacuum lines for cracks, leaks, or loose connections. Check the check valve. STEP 2: Measure reference voltage at the sensor (should be 5.0V) and ground using a multimeter. STEP 3: Check signal voltage with ignition on. At atmospheric pressure, the value should be around 4.0-4.5V (depending on sensor type). Under vacuum, voltage should drop towards 0.5V-1.5V. STEP 4: Use a diagnostic tool to command the electric vacuum pump on (if equipped) and monitor live data for pressure changes. STEP 5: If wiring and vacuum integrity are okay, replace the pressure sensor and clear codes.

Powertrain (Engine & Gearbox)Standard EOBD

P0556 - Brake Booster Pressure Sensor Circuit Range/Performance

Q: What are the symptoms of fault code P0556?

• Check Engine Light (MIL) illuminated • Hard or stiff brake pedal • Increased braking distance • Dashboard message: 'Brake Assist Reduced' or 'Service Brake System' • Engine may enter Limp Mode (reduced power)

Last updated:1 February 2026

Related:Brake Booster Pressure Sensor, Vacuum Pump, Vacuum Hoses, Brake Booster Check Valve, Engine Control Module (ECM)

System

Powertrain (Engine & Gearbox)

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Technical Description

P0556 is set when the Engine Control Module (ECM) detects that the Brake Booster Pressure Sensor signal is outside of a predetermined range or is not correlating with engine vacuum or electric vacuum pump activity. This sensor monitors the vacuum levels within the brake booster to ensure adequate power assist is available for braking. If the voltage signal (typically 0.5V to 4.5V) is implausible or fails to respond to changes in vacuum, the ECM triggers the code. This may impact brake pedal feel and can trigger a 'Limp Mode' to prioritize vehicle safety.

Possible Causes

Vacuum leak in hoses or check valve leading to the brake booster

Defective brake booster pressure sensor

Wiring issues (corrosion or short circuit in connector)

Faulty electric vacuum pump (if equipped)

Internal brake booster failure (diaphragm leak)

Symptoms

Check Engine Light (MIL) illuminated

Hard or stiff brake pedal

Increased braking distance

Dashboard message: 'Brake Assist Reduced' or 'Service Brake System'

Engine may enter Limp Mode (reduced power)

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P0556 - Brake Booster Pressure Sensor Circuit Range/Performance

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P0556 - Brake Booster Pressure Sensor Circuit Range/Performance

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