What it means
Brake Master Cylinder Piston Position Sensor A Circuit Range/Performance: the signal is implausible or outside the valid 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. This code sets when the signal is implausible or outside the valid operating range, even if it is not simply too high or too low.
Common causes
Most likely first.
- Sensor reading drifted
- Sensor element aged beyond calibration window
- Sensor mounting loose - signal damping
Symptoms
- ABS warning light on
- ESC/stability control may be disabled
How to diagnose it
- Scan the brake module and review freeze-frame data and live sensor A readings while the piston position changes to identify when the signal becomes implausible.
- Inspect sensor A's connector and wiring for loose terminals, corrosion, damaged insulation, or movement that could interrupt the signal.
- Check that sensor A is mounted securely and has not shifted, since loose mounting can damp or distort the reported movement.
- Compare sensor A's response with the manufacturer's expected range and procedure, checking its circuit and mounting before condemning the sensor.
- If the circuit and mounting are sound, test sensor A and the brake module input according to the manufacturer's procedure.
Common fixes
- Secure sensor A's mounting and correct its position according to the manufacturer's procedure.
- Clean and secure the connector or repair damaged wiring that makes the reading erratic.
- Replace sensor A if its reading remains outside the valid range after mounting and circuit checks.
- Repair the brake module only if its input is confirmed faulty.
Can I keep driving?
The ABS and stability systems may be disabled. Avoid high speeds and hard braking, and have the fault checked promptly because those systems may not respond as intended during a stop.
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.