Cause
Faulty brake booster pressure sensor
Monitors the vacuum or pressure available to assist the brakes, so the vehicle knows whether the driver's effort is being multiplied as it should be.
Its importance comes from what happens without assistance: the brakes still work, and they need several times the pedal effort. A driver who has never felt an unassisted pedal will often conclude the brakes have failed when they have not — which is precisely why the vehicle watches for it and warns.
On modern engines the assistance is less abundant than it used to be. A throttled petrol engine once produced plentiful manifold vacuum; direct injection, turbocharging and stop-start have all reduced it, so many vehicles now carry an electric vacuum pump. That pump is a leading cause of trouble here — it is a wear item, and a failing one leaves pressure the sensor correctly reports as insufficient.
So the first question is whether the sensor is wrong or the vacuum genuinely is, and they lead to completely different repairs. The clue is available to the driver: a pedal that has become noticeably harder, or that goes hard after two or three presses with the engine off, is a real vacuum problem rather than a sensor reporting one.
A perished booster hose or its one-way valve is the cheap end of that, and worth checking before anything else.
Typically a moderate job.
Codes this can cause (11)
Codes where this is a likely explanation first, rather than merely a possible one.
- C0047 Brake Booster Pressure Sensor Likely for this code Moderate
- C053D Brake Pressure Sensor A Range/Performance Likely for this code Moderate
- C0542 Brake Pressure Sensor B Circuit Low Likely for this code Moderate
- C0543 Brake Pressure Sensor B Circuit High Likely for this code Moderate
- P0555 Brake Booster Pressure Sensor Circuit Likely for this code High
- P0556 Brake Booster Pressure Sensor Circuit Range/Performance Likely for this code High
- P0557 Brake Booster Pressure Sensor Circuit Low Input Likely for this code Moderate
- P0558 Brake Booster Pressure Sensor Circuit High Input Likely for this code Moderate
- P0559 Brake Booster Pressure Sensor Circuit Intermittent Likely for this code Moderate
- U0418 Invalid Data Received From Brake System Control Module A Likely for this code High
- P258B Vacuum Pump Control Circuit A Range/Performance High
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