C0634Brake Pressure Sensor A/B Correlation
- Generic OBD-II code · Chassis
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- Two brake pressure sensors that see the same hydraulic pressure should report nearly the same value; C0634 means sensors A and B disagree beyond the allowed margin, even if each signal is valid on its own. Which sensor bears which letter depends on the maker. GM's 2019 brake module tables show the method but file it under C0574: two secondary circuit sensors are compared, with pedal travel as third reference, and a gap above 5 bar for 100 ms sets the fault, Type A with MIL. A Kelsey-Hayes patent places two elements on one diaphragm and disables hydraulic brake assist with a warning lamp when they differ, and a Lucas patent notes that two sensors only show that one is wrong, not which. No maker publishes C0634.
- What you may notice
- ABS, stability or brake system warning lamp on
- Brake assist or driver assistance functions with automatic braking off
- Likely causes
- LikelyOne sensor drifting in offset or gain
- PossibleFault in the brake control unit with integrated sensors, often replaced as a unit
- RareAir or a blocked passage making the sensors see different pressures
- RareBrake module software or calibration needing an update
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €50 – €2,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.