What it means
The control module compares brake pressure sensor A and B readings. It sets this code when the readings differ more than the allowed amount.
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 pressure sensors A and B provide pressure readings that the control module compares when managing braking and stability functions. C0634 is set when those two readings differ beyond the allowed correlation, suggesting that a sensor, its circuit, or the hydraulic pressure information is unreliable.
Common causes
Most likely first.
- One brake pressure sensor faulty or out of calibration
- Damaged wiring or poor connection to a sensor
- Hydraulic pressure issue affecting the readings
- Control module fault
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
How to diagnose it
- Read the ABS module freeze-frame data and check live readings from brake pressure sensors A and B with the pedal released and during gradual pedal application.
- Compare the two sensor readings for a consistent mismatch or an abrupt change, using the manufacturer's expected relationship rather than an assumed value.
- Inspect the wiring and connectors for both sensors for looseness, corrosion, fluid contamination, or damage.
- Check for other brake hydraulic or pressure-sensor codes that could explain why the readings disagree.
- Test each sensor circuit and sensor response, then assess hydraulic pressure and module inputs according to the manufacturer's procedure.
Common fixes
- Repair a loose, damaged, or corroded connection at sensor A or B.
- Repair a wiring fault affecting either pressure sensor circuit.
- Replace or recalibrate the sensor shown by testing to be inaccurate.
- Address a confirmed hydraulic pressure fault or module input problem.
Can I keep driving?
With pressure sensors A and B disagreeing, ABS and stability functions may be limited and braking assistance may be reduced. Avoid hard braking and high-speed driving; if pedal feel or braking performance changes, stop safely and seek prompt brake-system service.
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.