Chassis, generic

C0594 Brake Booster Motor A Performance

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C0594 judges whether booster motor A moves as commanded, not its wiring. GM's 2016/2019 hybrid booster flags a motor turning against the commanded direction (under 15 rad/s while more than 25 rad/s is requested), more than seven direction reversals, oscillation with the brake applied, or unintended backward rotation above 40 rad/s. In the XT4 unit and the 2023/2025 EBCM tables the plunger must reach its end position at start-up; the code also sets if the motor test finds a hardware fault, if pressure sensor 2 stays at or below 10 bar while the motor moves, or if modelled and measured pressure differ by over 40 bar for 0.2 s. All Type A with MIL. Mitsubishi files C18E1-49 (motor not operating normally) of the Outlander PHEV as C0594 with MIL.

Likely causes

Binding gear, spindle or plunger inside the booster
Worn or damaged booster motor A
Air or leakage in the hydraulic circuit so pressure does not follow motor travel
Faulty rotor position feedback causing wrong direction or oscillation

Symptoms

  • Loss of brake assist with a heavy pedal
  • Check engine and brake warning lamps on
  • Booster noise or pedal vibration before the fault sets

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€250–3,000
Time1–4 h

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

Parts involved

brake booster motor aelectric brake booster

Related codes

C0590C0591C058AC05C2C0582

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com, mitsubishitechinfo.com.