What it means
Master cylinder piston position sensors A and B disagree beyond plausible deviation.
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 compares the two master-cylinder piston-position signals to check whether they describe a plausible piston movement. This code sets when sensor A and sensor B disagree beyond the allowed correlation, which can make the module limit ABS or stability-control functions.
Common causes
Most likely first.
- Sensor A or B faulty or drifting
- One sensor misadjusted or not calibrated
- Wiring or connector fault affecting one signal
- Mechanical issue affecting piston movement
Symptoms
- ABS warning light on
- ESC/stability control may be disabled
How to diagnose it
- Read the brake-module freeze-frame and live data for both piston-position sensors to see whether their readings diverge at rest or as the pedal is pressed.
- Check the master-cylinder area and sensor connectors for loose plugs, corrosion, damaged wires, or signs that either sensor has shifted.
- Compare both sensor signals through a slow pedal application and release, looking for one signal that drops out, drifts, or fails to track piston movement.
- Test each sensor's supply, ground, and signal wiring for opens, shorts, or excess resistance, following the manufacturer's specification.
- If both circuits test correctly, check for abnormal piston movement and verify whether the sensors need adjustment or calibration under the manufacturer's procedure.
Common fixes
- Repair a loose, corroded, or damaged sensor connector or wire.
- Adjust or calibrate the affected piston-position sensor if the manufacturer's procedure permits it.
- Replace the sensor that has a faulty or drifting signal.
- Repair the master cylinder or other mechanical fault causing abnormal piston movement.
Can I keep driving?
The vehicle may retain basic braking, but ABS or stability control can be limited or disabled. Drive cautiously, leave extra stopping distance, and avoid high speeds or slippery roads until the fault is repaired.
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.