Chassis, generic

C0591 Brake Booster Motor A Phase U-V-W Current Low

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Current sense amplifier or ADC offset drifted below zero
Cracked solder joint or damaged shunt in the power stage
Unexpected reverse current through a faulty bridge transistor

Symptoms

  • Loss of electric brake assist, hard pedal
  • Check engine and brake warning lamps after one drive
  • Regenerative braking switched off

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€200–3,000
Time1–4 h

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

Parts involved

brake booster motor abrake system control module

Related codes

C0590C0593C0596C0595

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