C0591Brake Booster Motor A Phase U-V-W Current Low
- Generic OBD-II code · Chassis
- Turns on the check-engine light
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- Loss of electric brake assist, hard pedal
- Check engine and brake warning lamps after one drive
- Regenerative braking switched off
- Likely causes
- LikelyCurrent sense amplifier or ADC offset drifted below zero
- PossibleCracked solder joint or damaged shunt in the power stage
- RareUnexpected reverse current through a faulty bridge transistor
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €200 – €3,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.