C0591 code meaning
Brake Booster Motor A Phase U-V-W Current Low
Chassis system Generic code C05xx codes Check engine light
What C0591 means
C0591 is the low counterpart: the current value of booster motor A lies below what is plausible, in practice a negative reading when the motor is at rest. In GM's 2016/2019 hybrid booster a leakage reading under −20 A counts while a half bridge is idle; the 2023/2025 EBCM tables set it when the offset taken from the internal shunt falls below −38 A for 0.2 s. The XT4 module is stricter: any idle offset under 0 A with the motor not actuated, under −36.1 A in general, or a phase current under −200 A for 0.3 s. All are Type A with MIL. A negative value at standstill points less to the motor than to the measuring chain: amplifier offset, shunt solder joints or the module's analog input. Mitsubishi's Outlander PHEV reports it as C18E1-18 with MIL and switches off assist.
C0591 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 C0591
- Loss of electric brake assist, hard pedal
- Check engine and brake warning lamps after one drive
- Regenerative braking switched off
Common causes of C0591
- Most likelyCurrent sense amplifier or ADC offset drifted below zero
- PossibleCracked solder joint or damaged shunt in the power stage
- Less likelyUnexpected reverse current through a faulty bridge transistor
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Brake booster motor aBrake system control module
What each part of C0591 means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 91 | The exact fault in that area. |
What to do next
- Note C0591 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 C0591
What does C0591 mean?
C0591 means "Brake Booster Motor A Phase U-V-W Current Low". C0591 is the low counterpart: the current value of booster motor A lies below what is plausible, in practice a negative reading when the motor is at rest. It is a generic code, so it means the same on every make.
Is it safe to drive with C0591?
Drive gently and only to a workshop. With C0591 the car can go into limp mode (reduced power) to protect itself.
Will C0591 fail an emissions test?
C0591 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 C0591 to fix?
Difficulty: workshop only. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will C0591 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.