Chassis, generic

C054A Vacuum Sensor A Range/Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

Vacuum sensor A delivers an in-range voltage that does not agree with what the booster should hold. Patents show the logic: GM compares booster vacuum with engine vacuum from the MAP sensor while the throttle is closed, because the two should track each other without braking, and flags a sensor biased high or low when the difference persists too long; Ford relies on the check valve and treats a booster pressure clearly above manifold pressure as sensor degradation. Mitsubishi's 2023 Outlander PHEV brake unit has a related check that faults a booster vacuum of about 0 kPa while the engine runs (C106D-01), though that may also mean a real leak. No maker documents C054A publicly; A-B assignment varies. Electrical faults are C0549, C054B and C054C.

Likely causes

Vacuum sensor drifted or contaminated (oil, moisture)
Real leak in hose, check valve or booster making the reading look wrong
Faulty comparison value, such as manifold pressure

Symptoms

  • Brake assist warning or reduced assist feel
  • Electric vacuum pump running more often than normal
  • Stop-start or cylinder deactivation inhibited

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–400
Time0.3–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

vacuum sensor a

Related codes

C0549C054BC054CC054EP0556

References: patents.google.com, patents.google.com, mitsubishitechinfo.com.