What it means
Brake master cylinder piston position sensor A signal voltage above 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
Brake master cylinder piston position sensor A reports piston movement to the brake control module, which uses the signal in brake-control calculations. This code sets when sensor A's signal is higher than the expected range.
Common causes
Most likely first.
- Signal wire shorted to voltage
- Sensor A internally faulty
- Connector corrosion or damage causing a high signal
- Control module fault
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
How to diagnose it
- Scan the brake module for related codes and review freeze-frame data and live sensor A readings to confirm whether the high signal is continuous or intermittent.
- Inspect sensor A's connector for corrosion, moisture, damaged terminals, or a poor fit that could disturb the signal.
- Check the sensor A harness for damaged insulation or contact with a powered wire, and gently move it while monitoring the live reading.
- Using vehicle wiring information, test sensor A's signal circuit for a short to voltage and check its supply and ground against the manufacturer's specification.
- If the wiring checks pass, test sensor A and the brake module input according to the manufacturer's procedure.
Common fixes
- Clean and secure sensor A's connector and repair damaged terminals.
- Repair chafed wiring or a signal wire shorted to voltage.
- Replace sensor A if it fails the manufacturer's test.
- Repair or replace the brake module only if its input is confirmed faulty.
Can I keep driving?
The ABS and stability systems may be disabled, and brake assist may be reduced. Avoid high speeds and hard braking; stop and arrange prompt inspection if pedal feel or braking changes.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.