P0556
Brake Booster Pressure Sensor Circuit Range/Performance.
- Generic
- Powertrain
- Generic code
- High severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 5
- AreaVehicle speed, idle control and auxiliary inputs
- 56
- FaultThe specific fault in that area
What it means
P0556 is the generic code for a range or performance fault in the brake booster pressure sensor circuit. The control unit compares the sensor's signal with what it expects from engine vacuum, other sensor inputs or operating conditions, and sets the code when the reading is implausible, stuck or does not change as it should. Exact strategy and sensor location vary by manufacturer and model.
Symptoms
- Warning light on (engine management light, and sometimes a brake or stability control warning)
- Hard or heavy brake pedal feel
- Brake pedal feels unusually soft or inconsistent
- Rough idle or stalling if a vacuum leak is involved
- Stop-start system disabled or behaving erratically
- Hissing noise from the engine bay or pedal area when braking
Possible causes
- Vacuum leak in the brake booster, its check valve or the vacuum hoselikely
- Faulty or drifting brake booster pressure sensorlikely
- Damaged, corroded or loose wiring or connector at the sensorpossible
- Weak vacuum supply from the engine or vacuum pump (where fitted)possible
- Failed or leaking booster diaphragmpossible
- Fault within the control unit or a software issuerare
How to diagnose
- Read the stored and pending codes and note the freeze frame data, including engine speed, load and any related brake or vacuum codes
- Visually inspect the booster vacuum hose, check valve and sensor connector for cracks, splits, chafing, corrosion or loose fit
- With the engine off, pump the brake pedal until it is hard, then hold light pressure while starting the engine; the pedal should sink slightly, and if not, suspect a vacuum or booster fault
- Listen for hissing at the booster, pedal and hose with the engine running and check for vacuum leaks
- Check the vacuum supply at the booster with a suitable gauge and compare with the manufacturer's specification, including the vacuum pump if fitted
- Check the sensor's supply voltage, earth and signal wiring for continuity, shorts and voltage drop against the wiring diagram
- View live sensor data with the engine off, idling and while braking, and check the reading is plausible and responds smoothly
- Check for applicable technical bulletins or software updates for the specific model and engine
How to fix
- Replace any leaking or damaged vacuum hose or faulty check valve
- Replace the brake booster pressure sensor if its readings are implausible and the wiring is sound
- Repair or replace damaged wiring and clean or renew corroded connectors
- Repair or replace the vacuum pump or address the cause of poor engine vacuum, as applicable
- Replace the brake booster assembly if the diaphragm is leaking
- Apply a manufacturer software update only if a known calibration fault applies to the vehicle
Can I keep driving?
High severity
Because the code may indicate reduced brake assistance, avoid driving until the braking system has been checked. If the pedal feels hard or heavy, do not drive the car; have it recovered or repaired.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.