Brake Booster Pressure Sensor Circuit Range/Performance

Possible causes

• CommonFailed brake booster pressure sensor
• CommonVacuum leak in the booster hose or its non-return valve
• CommonCorroded connector or damaged wiring
• PossibleFailed vacuum pump, on engines that use one
• PossibleFailed brake booster diaphragm
• PossibleLoss of reference voltage or earth

What you might notice

• Brake pedal harder than usual
• More pedal effort needed to stop
• Hissing from the pedal area
• Brake warning message
• Engine idle affected when braking

What to check, in order

1. Do the pedal test first. If assistance is genuinely missing, the sensor is telling the truth and the vacuum supply is the fault.
2. Test the pedal before driving. With the engine off, press the brake pedal several times to use up the stored vacuum, then start the engine with your foot on the pedal - it should sink slightly as assistance arrives. If it does not, assistance is genuinely missing.
3. Check the booster vacuum hose and its non-return valve for splits and looseness.
4. Inspect the sensor connector for corrosion and water.
5. Check the reference voltage and earth at the sensor.
6. On a diesel or a direct-injection petrol engine, check the vacuum pump - these engines make little vacuum of their own and depend on it.