What it means
The brake control module monitors the voltage signal from brake master cylinder piston position sensor B. It sets this code when the signal falls below 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
Brake master cylinder piston position sensor B reports piston movement to the brake control module. This code sets when sensor B's signal falls below the expected range, which can result from a short to ground, a poor circuit connection, or a faulty sensor.
Common causes
Most likely first.
- Signal wire shorted to ground
- Open or high-resistance signal or ground wiring
- Sensor B faulty or misadjusted
- Connector corrosion or poor contact
- Control module fault
Symptoms
- ABS warning light on
- ESC/stability control may be disabled
How to diagnose it
- Scan the brake module for related codes and review freeze-frame data and live sensor B readings to determine whether the low signal is constant or intermittent.
- Inspect sensor B's connector for corrosion, moisture, loose fit, damaged terminals, or a pin pushed back into the connector body.
- Check sensor B's harness for chafing or a broken wire, then gently move it while monitoring the live reading for changes.
- Using vehicle wiring information, test sensor B's signal for a short to ground and check the signal, supply, and ground circuits against the manufacturer's specification.
- If the wiring is sound, check sensor B's mounting and test the sensor and brake module input according to the manufacturer's procedure.
Common fixes
- Clean and secure sensor B's connector and restore any backed-out terminal.
- Repair an open, high-resistance, or shorted-to-ground section of sensor B wiring.
- Correct sensor B's mounting or adjustment if required by the manufacturer's procedure.
- Replace sensor B if it fails the manufacturer's test.
Can I keep driving?
ABS and stability control may be unavailable, and brake assist may be reduced. Avoid high speeds and hard braking, and stop safely if the pedal feels abnormal or braking performance is reduced.
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.