Chassis, generic

C0549 Vacuum Sensor A Circuit/Open

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

General circuit fault of vacuum sensor A, usually a broken or disconnected signal, ground or feed line. The sensor measures the vacuum in the brake booster; Bosch notes that its value lets the system run the electric vacuum pump, permits engine stop-start and supports ESP functions. Mitsubishi shows how a brake unit watches it on the 2023 Outlander PHEV: with power on and normal supply, an open or a short in the brake vacuum sensor circuit, or signal noise, is detected within a second and stored as C1073, after which brake assist is suspended; Mitsubishi does not map that fault to an SAE code. Which physical sensor is called A is manufacturer-specific, and no maker documents C0549 publicly. C054A to C054C cover plausibility, low and high signal.

Likely causes

Sensor plug loose or not latched after work in the engine bay
Broken signal, ground or supply wire
Internal open in the vacuum sensor

Symptoms

  • Brake assist warning or ABS lamp on
  • Stop-start no longer switching off the engine

Repair estimate

DifficultyEasyDIY possible
Parts and labour€20–250
Time0.2–1.5 h

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

Parts involved

vacuum sensor asensor wiring

Related codes

C054AC054BC054CC054DC0547

References: mitsubishitechinfo.com, bosch-mobility.com, hella.com.