C0594 code meaning
Brake Booster Motor A Performance
Chassis system Generic code C05xx codes Check engine light
What C0594 means
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.
C0594 at a glance
- Check engine lightYes, usually on
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyWorkshop only
- Typical labor1 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of C0594
- Loss of brake assist with a heavy pedal
- Check engine and brake warning lamps on
- Booster noise or pedal vibration before the fault sets
Common causes of C0594
- Most likelyBinding gear, spindle or plunger inside the booster
- PossibleWorn or damaged booster motor A
- PossibleAir or leakage in the hydraulic circuit so pressure does not follow motor travel
- Less likelyFaulty rotor position feedback causing wrong direction or oscillation
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Brake booster motor aElectric brake booster
What each part of C0594 means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 94 | The exact fault in that area. |
What to do next
- Note C0594 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about C0594
What does C0594 mean?
C0594 means "Brake Booster Motor A Performance". C0594 judges whether booster motor A moves as commanded, not its wiring. It is a generic code, so it means the same on every make.
Is it safe to drive with C0594?
Drive gently and only to a workshop. With C0594 the car can go into limp mode (reduced power) to protect itself.
Will C0594 fail an emissions test?
C0594 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is C0594 to fix?
Difficulty: workshop only. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will C0594 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.