Code
C0591
Generic
C — Chassis
Brake Booster Motor A Phase U-V-W Current Low
Views:
UK: 18
EN: 26
RU: 17
AI status
Completed
Completed
100%
Causes
- Open or high resistance in phase wiring (U, V or W)
- Disconnected or corroded connector or pin
- Failed motor winding (open circuit)
- Faulty motor control driver (ABS/BCM/EBB module output stage)
- Blown fuse or faulty relay feeding the motor supply
- Low battery voltage or poor battery/ground connection
Symptoms
- Reduced or no brake boost (harder brake pedal, increased pedal travel)
- Brake warning lamp and/or ABS/ESC warning lamp illuminated
- Diagnostic trouble code C0591 stored
- Brake booster motor may not run or sounds weak/abnormal when commanded
- Possible limp or reduced function brake assist message
What to check
- Read all stored DTCs and freeze frame / live data with a capable scan tool
- Verify battery voltage and main grounds with key on and during motor request
- Visually inspect motor connector and wiring harness for damage, corrosion, or loose pins
- Check fuses and relays related to the brake booster motor and replace if blown or suspect
- Command the brake booster motor using a scanner and observe motor operation and sounds
- Measure phase continuity and resistance between U, V, W and motor ground/module
Signal parameters
- Motor phase currents (U, V, W) — compare to manufacturer nominal and threshold for low current
- Motor supply voltage (battery / motor supply) during command
- Control module command signal (PWM duty cycle and frequency to motor driver)
- Motor winding resistance (Ω) between phases and to ground
- Module internal fault flags and driver temperature or output stage status
- Vehicle speed/ignition state and related inhibit signals that may prevent motor activation
Diagnostic algorithm
- Connect a diagnostic scanner, record trouble codes, freeze frame data and live parameters related to the brake booster motor.
- Verify battery state of charge and resting voltage; recharge or replace battery if below specification. Check engine/vehicle grounds.
- Visually inspect the motor connector and harness for damage, bent pins, corrosion or water ingress; repair any issues and clear codes for retest.
- Check fuses and relays for motor power; verify motor supply voltage at the motor connector while commanding the motor with a scan tool.
- With ignition off, measure DC resistance between motor phase terminals (U‑V, V‑W, U‑W). Compare to spec; infinite or very high resistance indicates open winding.
- With scanner commanding the motor, use a DC clamp meter to measure phase currents and compare to nominal. Confirm whether current is below threshold on one or all phases.
- If phase currents are low but supply voltage and wiring continuity are good, suspect driver/module failure. Verify control outputs (PWM) from module with an oscilloscope if available.
- If wiring and module outputs check good but motor current remains low, replace or bench‑test the motor assembly per manufacturer procedure.
- If a module or driver is replaced, program/configure per manufacturer instructions. Clear codes and perform functional test of brake assist and verify no code returns.
- If intermittent, perform wiggle tests on harness while commanding motor and monitor currents to find intermittent opens or shorts.
Likely causes
- Damaged connector or broken wire in one of the U/V/W phase leads
- Open motor winding (internal motor failure)
- Blown motor supply fuse or stuck relay
- Faulty power stage (MOSFETs) in the brake booster control module
- Low battery voltage or poor ground at time of activation
Fault status
Status
Brake Booster Motor A phase U‑V‑W current below expected threshold. Check motor, supply, wiring and controller output. Brake assist may be reduced.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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