P0559 - Brake Booster Pressure Sensor Circuit High
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0559 is stored when the Engine Control Module (ECM) detects a voltage signal from the brake booster pressure sensor that exceeds the calibrated limit (typically above 4.8V). This sensor monitors the vacuum levels within the brake booster to ensure adequate power assist is available. In vehicles equipped with start-stop systems or turbocharged engines, this input is critical for managing the auxiliary electric vacuum pump. A high circuit condition often points to an open circuit on the ground side, a short to voltage on the signal wire, or a failed sensor. This fault may disable start-stop functionality and alter brake pedal feel.
Possible Causes
Faulty brake pressure sensor (internal short)
Open circuit in the sensor ground wire
Short to 5V reference on the signal wire
Defective Engine Control Module (ECM) or connector corrosion
Symptoms
Recommended Solution
- STEP 1: Visually inspect the connector at the pressure sensor (located on the brake booster). Look for corrosion or backed-out pins.
- STEP 3: Check ground integrity. Resistance between sensor ground and chassis ground should be < 0.5 Ohms.
- STEP 4: Use a scan tool to monitor 'Brake Booster Pressure' live data. If the value is stuck at maximum despite vacuum presence, replace the sensor.
- STEP 5: Clear codes and perform a test drive including multiple braking cycles.
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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