Faulty brake booster pressure sensor giving a low output; Wiring damage, chafing or short to earth in the sensor signal circuit; Corroded, loose or contaminated connector terminals at the sensor; Open circuit or short in the sensor 5 volt supply or earth wire; Vacuum leak or failed vacuum hose or non-return valve causing a genuinely low booster pressure reading; Faulty vacuum pump or booster problem on vehicles that use one; Fault inside the control unit.

P0557

Brake Booster Pressure Sensor Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
5: AreaVehicle speed, idle control and auxiliary inputs
57: FaultThe specific fault in that area

What it means

P0557 is the generic code for a low signal from the brake booster pressure sensor circuit. The control unit monitors the sensor's voltage, which reflects vacuum in the brake booster, and sets the code when that voltage falls below the expected range, usually from a short to earth, an open circuit, or a failed sensor. How the signal is used, such as for engine idle control or brake assist, varies by vehicle.

Symptoms

Warning light on (engine management light, and possibly a brake or stability control warning)
Hard or heavy brake pedal
Unstable or rough idle, or stalling when braking
Reduced braking assistance on some vehicles
Increased pedal effort at low engine speed
No noticeable symptoms other than the stored code

Possible causes

Faulty brake booster pressure sensor giving a low outputlikely
Wiring damage, chafing or short to earth in the sensor signal circuitlikely
Corroded, loose or contaminated connector terminals at the sensorlikely
Open circuit or short in the sensor 5 volt supply or earth wirepossible
Vacuum leak or failed vacuum hose or non-return valve causing a genuinely low booster pressure readingpossible
Faulty vacuum pump or booster problem on vehicles that use onepossible
Fault inside the control unitrare

How to diagnose

Read all stored and pending codes and the freeze frame data to see engine speed, load and conditions when the fault set.
Look for other related codes such as sensor supply voltage faults, as these point towards a shared circuit problem.
Inspect the sensor, its connector and the nearby wiring for damage, corrosion, water ingress, or chafing.
Check the vacuum hose and non-return valve between the engine or pump and the booster for splits, loose fittings and correct operation.
Compare the live sensor reading with the expected value for engine off and engine running, following the manufacturer's data for the model.
Check the sensor supply voltage and earth at the connector with a multimeter, using the wiring diagram for the vehicle.
Test the signal wire for a short to earth or an open circuit between the sensor and control unit.
Check the vacuum pump and booster operation where fitted, as the procedure depends on the model and engine.

How to fix

Replace the brake booster pressure sensor if its signal is out of range and the wiring is sound.
Repair or replace damaged wiring and clean or renew corroded connector terminals.
Restore the correct supply or earth to the sensor by repairing the circuit fault.
Replace split vacuum hoses or a faulty non-return valve.
Repair or replace the vacuum pump or booster if testing shows it cannot produce or hold vacuum.
Clear the code after repair and road test to confirm it does not return.

Can I keep driving?

Medium severity

The car can usually be driven short distances, but braking assistance may be reduced and the pedal can feel heavy, so get it checked soon. If the brake pedal feels hard or braking is affected, stop driving until it is inspected.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.