C004C: Brake Booster Motor A Position Sensor Exceeded Learning Limit

What it means

Brake booster motor A position sensor learned offset exceeded the allowed range. Boost control tracks commanded vs. actual manifold pressure; persistent deviation means wastegate, vanes, hose or actuator are not delivering the requested pressure.

At a glance

• Usually lights the ABS or stability-control warning light

How it works

The brake booster motor A position sensor reports the position of booster motor A so the control system can track its movement and learned offset. C004C sets when that learned offset exceeds its allowed range, which can follow an incomplete calibration, a sensor or circuit fault, or a motor mechanism problem.

Common causes

Most likely first.

1. Brake booster motor position sensor fault
2. Position sensor wiring or connector fault
3. Learning or calibration not completed after service
4. Brake booster motor or mechanism fault

Symptoms

• ABS warning light on
• ESC/stability control deactivated
• Possible reduced brake assist
• Speedometer may behave erratically

How to diagnose it

1. Read freeze-frame data and the available calibration or position values to confirm the motor A learning-limit fault and the conditions when it set.
2. Check service history for recent booster or motor work and whether the required learning procedure was completed.
3. Inspect the motor A position sensor connector and wiring for loose pins, corrosion, chafing, or damage.
4. Compare commanded motor A position with live sensor feedback and check the sensor's power, ground, and signal circuits against the manufacturer's specification.
5. If the circuit passes, test motor A movement and complete the manufacturer's learning procedure; investigate the mechanism if its position still cannot be learned.

Common fixes

• Repair a loose connector or damaged motor A position sensor wiring.
• Complete the required motor A position learning or calibration procedure after service.
• Replace the motor A position sensor if its signal fails testing.
• Repair or replace the booster motor or mechanism if it cannot move through the required range.
• Repair the brake control module only if its position input or control output is confirmed faulty.

Can I keep driving?

ABS or stability assistance may be disabled, and brake assist could be reduced. Avoid driving if the pedal feels different or braking is impaired; otherwise drive cautiously and have the booster system checked promptly.