Chassis, generic

C0634 Brake Pressure Sensor A/B Correlation

Check engine lightUsually off
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

One sensor drifting in offset or gain
Fault in the brake control unit with integrated sensors, often replaced as a unit
Air or a blocked passage making the sensors see different pressures
Brake module software or calibration needing an update

Symptoms

  • ABS, stability or brake system warning lamp on
  • Brake assist or driver assistance functions with automatic braking off

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€50–2,500
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

brake pressure sensor abrake pressure sensor b

Related codes

C053DC0541C0574C056B

References: gsi.ext.gm.com, patents.google.com, patents.google.com.