Chassis, generic

C0637 Brake Pressure Sensor G Circuit Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C0637 reports that sensor G delivers less voltage than its lowest valid reading. Brake pressure sensors keep a live zero at rest, so a signal near 0 V means a fault rather than zero pressure: Saab's 9-5 ESP flags a pressure signal under 0.25 V lasting 0.1 s, and GM's 2016 hybrid brake module treats under 0.3 V for 0.1 s on its left rear wheel pressure sensor (sensor D there) as a short to ground, Type A. Common causes are a signal wire chafed through to the body, a 5 V reference pulled down, or a failed sensor. Which component G is stays open; in a seven-sensor unit like the Lucas/ZF design it would be one of the wheel, supply or push-through sensors. G usually sits in the hydraulic block, so a sensor defect often means replacing that block.

Likely causes

Signal wire chafed and shorted to ground
5 V sensor reference shorted to ground or interrupted
Sensor G defective, often only replaceable with the hydraulic unit

Symptoms

  • ABS and ESC warning lamps on
  • Live data shows no pressure even when the pedal is pressed

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–1,800
Time0.5–4 h

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

Parts involved

brake pressure sensor gsensor wiring

Related codes

C0635C0636C0638C063BC05C6C0575

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