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DTC codes / C05xx · Brakes, steering and suspension / C05C3

C05C3Brake Booster Motor A Supply Circuit

C05C3 indicates that the antilock brake system (ABS) control module has detected a fault in the power supply circuit for the electric brake booster motor, specifically the 'A' circuit. The module monitors voltage and current on this circuit and sets the code when the signal falls outside the expected range, which can prevent the booster motor from assisting brake application.

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

Symptoms you may notice

  • Illumination of the ABS warning light, brake system warning light, or both on the dashboard
  • A hard brake pedal that requires significantly more effort to stop the vehicle
  • Reduced or absent power brake assist, especially during the first few pedal applications after start-up
  • Audible warning chime or message on the driver information center indicating a brake system fault
  • Often no symptoms besides the warning light, with normal pedal feel in some cases

Common causes, most likely first

  1. Blown fuse or tripped circuit breaker dedicated to the brake booster motor supply circuit
  2. Damaged, corroded, or disconnected wiring or connector at the booster motor or ABS module
  3. Failed brake booster motor (open winding, worn brushes, or internal short)
  4. Faulty ABS control module or power supply relay internal to the module
  5. High resistance in the ground circuit for the booster motor, causing voltage drop under load
  6. Low battery voltage or poor charging system performance affecting the supply circuit

How to diagnose and fix it

  1. Verify battery state of charge and charging system output; a weak battery can set this code
  2. Check the fuse and relay for the brake booster motor supply circuit and replace if blown or faulty
  3. Inspect the wiring harness and connector at the booster motor for chafing, corrosion, or loose pins
  4. Measure voltage and resistance on the supply circuit under load to identify excessive voltage drop
  5. Perform a bidirectional actuator test of the booster motor with a scan tool to confirm motor operation
  6. If the motor fails to run with good power and ground, replace the brake booster motor assembly
  7. If the circuit tests good but the code persists, diagnose or replace the ABS control module

What the booster motor circuit hides

  • A weak battery or failing alternator can set C05C3 even when the booster motor and wiring are healthy, because the module sees low voltage during the motor self-test at start-up.
  • Corrosion inside the booster motor connector is a frequent cause that is missed when technicians only check for a blown fuse and then replace the motor.
  • The code can be stored intermittently by a momentary voltage drop during engine cranking, so always check for a history of low-voltage events before condemning the motor.
  • Some vehicles use a shared ground point for the booster motor and other chassis electronics; a loose ground can set C05C3 along with unrelated codes.
  • Replacing the ABS module without first confirming the booster motor circuit is intact can lead to a repeat failure and unnecessary expense.

Can I keep driving with C05C3?

If the brake pedal feels hard or assist is reduced, avoid driving the vehicle because stopping distances increase significantly and emergency braking may be compromised. If the pedal feels normal and only the warning light is on, drive cautiously to a repair facility without delay.

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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