Chassis, generic

C0638 Brake Pressure Sensor G Range/Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

Here sensor G produces a voltage inside its window, but the pressure it reports disagrees with what the system expects. Lucas/ZF describe the classic test for multi-sensor EHB units: after the driver brakes to a stop, the isolation valve stays closed and all wheel valves open, so three or more sensors share one pressure; the odd one out is the faulty sensor, with size and direction of its error. Two sensors alone would only reveal that something is wrong. A Bosch patent learns offsets while driving, flags a fault once the correction grows beyond a threshold and keeps ABS lamp and ESC telltale lit until a later good check. No manufacturer documents C0638; GM's PIP5094 contains the number from an earlier GM scheme. Causes: drift, trapped pressure, a sticking valve.

Likely causes

Zero point or gain of sensor G drifted
Pressure trapped in the circuit during the comparison or zero learn
Valve sticking so the sensor does not see the shared test pressure

Symptoms

  • ABS lamp and ESC telltale on
  • Stability control reduced or off, regenerative braking possibly limited

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–1,800
Time0.5–4 h

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

Parts involved

brake pressure sensor g

Related codes

C0635C0636C0637C063CC05C5C0634

References: patents.google.com, patents.google.com, patents.google.com, static.nhtsa.gov.