P0557 - Brake Booster Pressure Sensor Circuit Low
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0557 is a generic OBD-II code indicating that the Engine Control Module (ECM) has detected a voltage signal from the Brake Booster Pressure Sensor that is below the calibrated minimum threshold (typically below 0.2V). This sensor monitors the vacuum level within the brake booster to ensure adequate power assist is available. On vehicles equipped with start-stop systems or turbocharged engines, this signal is critical for determining when to activate the auxiliary vacuum pump or restart the engine. A 'Circuit Low' condition usually points towards a short to ground or an open circuit in the 5V reference line.
Possible Causes
Faulty Brake Booster Pressure Sensor (internal short)
Wiring issues (short to ground on the signal circuit)
Open circuit in the 5V reference line from the ECU
Faulty Engine Control Module (ECM)
Symptoms
Recommended Solution
Workflow
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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