C05D1: Brake Master Cylinder Piston Position Sensor Supply Circuit

What it means

The brake control module monitors the supply circuit for the master cylinder piston position sensors. It sets this code when the shared supply voltage is outside the expected range.

At a glance

• Usually lights the ABS or stability-control warning light
• The engine or gearbox may go into limp mode

How it works

The brake control module supplies power to the master-cylinder piston-position sensors through a shared supply circuit. This code sets when that supply is outside its expected range, potentially removing or corrupting both sensor readings.

Common causes

Most likely first.

1. Open or short in the sensor supply wiring
2. Blown fuse or faulty supply from the control module
3. Loose or corroded connector
4. A sensor internally shorted and pulling down the supply

Symptoms

• ABS warning light on
• ESC/stability control may be disabled

How to diagnose it

1. Read brake-module freeze-frame and live data for the sensor supply and both piston-position signals to determine whether the supply fault is present now or intermittent.
2. Inspect the shared supply wiring and sensor connectors for chafing, pin damage, corrosion, or a loose connection.
3. Check the relevant fuse and verify supply and ground at the sensor connector using the manufacturer's specification.
4. With power isolated as directed by the service information, check the supply wire for an open or short to ground or power.
5. If the wiring and fuse are sound, isolate the sensors one at a time as specified by the manufacturer to find whether one is pulling down the shared supply; assess module output only after those checks.

Common fixes

• Clean and secure a loose or corroded sensor connector.
• Repair an open, shorted, or damaged section of shared supply wiring.
• Replace a blown fuse after finding and correcting the cause.
• Replace a sensor that internally shorts the shared supply.
• Repair the brake control module supply fault if wiring, fuse, and sensors check out.

Can I keep driving?

Basic braking may remain available, but ABS or stability control may be disabled. Drive cautiously and avoid conditions that rely on those systems until the supply fault is fixed.