P0559

What it means

P0559 is the generic code for an intermittent fault in the brake booster pressure sensor circuit. The control unit saw the sensor signal drop out, jump or go implausible for short periods, rather than staying permanently out of range. The sensor monitors vacuum or pressure in the brake booster, and the exact system that uses it varies by model.

Symptoms

• Warning light on (engine management light, and on some cars a brake or stability control light)
• Brake pedal feel that changes between hard and normal, or reduced brake assistance
• Rough idle or stalling on some vehicles where booster vacuum data is used for engine control
• Stop-start system or other brake-related functions not working at times
• Fault appearing and disappearing, often on bumps or after wet weather

Possible causes

• Loose, corroded or damaged connector at the brake booster pressure sensorlikely
• Chafed, broken or high-resistance wiring between the sensor and the control unitlikely
• Failing brake booster pressure sensor with intermittent internal faultpossible
• Leak in the vacuum hose, non-return valve or booster causing an unstable readingpossible
• Poor supply voltage or earth to the sensor, such as a weak battery or poor earth connectionpossible
• Fault in the control unit or its internal sensor circuitrare

How to diagnose

1. Read all stored and pending codes and freeze frame data, and note when and under what conditions the fault set
2. Check the brake pedal feel with the engine off and on, and listen for a hiss from a vacuum leak at the pedal or booster
3. Inspect the sensor connector and nearby wiring for corrosion, water ingress, loose pins, chafing or heat damage
4. Inspect the vacuum hose and non-return valve for cracks, splits, poor fit or damage
5. Check battery condition and charging voltage, as low or unstable voltage can cause intermittent sensor faults
6. Watch the live sensor reading while wiggling the connector and harness, and look for dropouts or spikes
7. Check the sensor supply, signal and earth circuits against the manufacturer's wiring diagram and specification
8. Compare the live reading with the expected values for your model, as the sensor type and operating range vary

How to fix

• Clean, repair or replace the damaged connector or terminals
• Repair or replace the faulty wiring section, using proper soldered or crimped and sealed joints
• Replace the brake booster pressure sensor if it fails testing
• Replace any leaking vacuum hose or faulty non-return valve, or rectify the booster fault
• Repair the poor earth or supply fault, and correct any battery or charging problem

Can I keep driving?

Nearby codes

• P0558GenericBrake Booster Pressure Sensor Circuit High
• P055AGenericEngine Oil Pressure Sensor/Switch B Circuit