What it means
Master cylinder piston position higher than expected — typically too much pedal travel for the commanded brake pressure.
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 uses master-cylinder piston position, along with brake-pressure information, to assess hydraulic response to pedal input. This code sets when the piston position is higher than expected for the commanded brake pressure, which can indicate excess pedal travel or a sensing error.
Common causes
Most likely first.
- Air in the hydraulic brake system
- Brake fluid leak or low fluid level
- Internal master cylinder bypass or seal wear
- Fault in brake pressure or piston position sensing
Symptoms
- ABS warning light on
- ESC/stability control may be disabled
How to diagnose it
- Read freeze-frame and live brake data, including piston position and pressure, and note whether the high position occurs at rest or during pedal application.
- Check the brake-fluid level and inspect the master cylinder, brake lines, hoses, and wheel areas for leaks before driving further.
- With the vehicle safely stationary, note whether the pedal feels unusually soft, sinks, or travels farther than normal; do not continue if braking feels impaired.
- If there is no leak, have the hydraulic system checked for air and bleeding performed using the manufacturer's procedure.
- If hydraulic condition is sound, compare the piston-position and pressure sensor readings with the manufacturer's diagnostic procedure, then assess the master cylinder for internal bypass.
Common fixes
- Repair any brake-fluid leak and restore the specified fluid level.
- Bleed air from the hydraulic system using the manufacturer's procedure.
- Repair or replace a faulty piston-position or pressure sensor or its wiring.
- Replace a master cylinder with confirmed internal bypass or worn seals.
Can I keep driving?
Do not drive if the pedal is soft, sinks, has excessive travel, or braking is reduced; arrange recovery instead. A hydraulic fault can worsen and increase stopping distance, while ABS or stability control may also be unavailable.
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.