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DTC codes / C05xx · Brakes, steering and suspension / C0591

C0591Brake Booster Motor A Phase U-V-W Current Low

C0591 indicates that the brake booster motor's phase U, V, or W current draw is lower than expected, meaning the motor is not receiving or is not drawing the commanded current on at least one of its three electrical phases. The control module monitors the current in each motor phase and sets this code when the measured current falls below the calibrated threshold, typically indicating an open circuit, high resistance, or a faulty motor winding.

SystemChassis
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaBrakes, steering and suspension

Symptoms you may notice

  • Illumination of the brake system warning light and possibly the ABS (Anti-lock Braking System) or stability control warning light.
  • Often no symptoms besides the warning light, especially if the brake booster motor is only intermittently affected.
  • Reduced or absent power brake assist, resulting in a hard brake pedal that requires significantly more effort to stop the vehicle.
  • A noticeable delay or lack of response when pressing the brake pedal, particularly during the first application after start-up.
  • Audible clicking or no sound from the brake booster motor area when the brake pedal is pressed, indicating the motor is not running.
  • In some vehicles, the system may default to a failsafe mode that limits vehicle speed or disables certain driver-assist features.

Common causes, most likely first

  1. Open or high-resistance wiring in the brake booster motor phase circuits, including damaged harnesses, corroded connectors, or broken pins.
  2. Faulty brake booster motor with an open or shorted winding on one or more phases, often due to internal wear or overheating.
  3. Poor electrical connection at the motor or control module, such as loose terminals or contamination in the connector.
  4. Failed brake booster control module or power supply relay that does not deliver proper voltage or current to the motor.
  5. Blown fuse or fusible link in the brake booster motor power circuit, though this often affects all phases rather than a single phase.
  6. Mechanical binding or seized brake booster mechanism that prevents the motor from turning, causing low current draw on all phases.

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related codes in the ABS (Anti-lock Braking System), stability control, and powertrain control modules to understand the full fault context.
  2. Inspect the brake booster motor harness and connectors for visible damage, corrosion, or loose pins, and perform a wiggle test while monitoring live data for phase current.
  3. Measure resistance of each motor phase winding at the motor connector and compare to specifications; an open or significantly unbalanced reading indicates a faulty motor.
  4. Check the power and ground circuits to the brake booster control module, including fuses and relays, to ensure the module is receiving proper voltage and ground.
  5. Perform a voltage drop test on the phase circuits under load to identify high-resistance connections that may not show up in a simple resistance check.
  6. If wiring and power supplies are good, substitute a known-good brake booster motor or control module to confirm the faulty component before replacement.
  7. After repair, clear the code and perform a brake system calibration or relearn procedure as required by the manufacturer, then test-drive to verify the fix.

When phase current looks low but isn't

  • A low current reading on one phase can be caused by a poor ground connection at the motor housing rather than a faulty winding, so always check grounds before condemning the motor.
  • Some vehicles use a shared power supply for all three phases and switch the ground side; a single open ground path can mimic a phase-specific fault and set this code.
  • The brake booster motor may only run during certain conditions such as initial key-on or during active brake assist, so the fault may not be present during a static test and requires live data monitoring during a road test.
  • A weak or failing battery can cause the control module to misread phase currents, especially during cranking, so verify battery health and charging system output before deeper diagnostics.
  • This code can be set by a mechanical issue such as a sticking brake booster pushrod or a binding pedal assembly that prevents the motor from turning freely, which is often overlooked when focusing only on electrical causes.

Can I keep driving with C0591?

If the brake pedal feels normal and the warning light is the only symptom, you may drive cautiously to a repair shop, but avoid heavy braking or emergency stops. If the pedal is hard or braking performance is reduced, do not drive the vehicle because loss of power brake assist can significantly increase stopping distance and make the vehicle unsafe to operate.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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