Chassis, generic

C0646 Brake Pressure Sensor L Circuit High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C0646 means the signal of brake pressure sensor L lies above the band a real pressure can produce. A Kelsey-Hayes patent spells out the logic for a bridge-type brake sensor: a healthy output stays between zero and the supply voltage, so a reading equal to supply points to an open circuit, not to maximum pressure. A high code can therefore hide a lost ground or a broken signal line as well as a short to 5 V. Truck trailer brakes show where extra analog pressure sensors appear: WABCO's Trailer EBS D feeds an optional external brake pressure sensor from the modulator and reads it on a 0–5 V analog input. No carmaker documents C0646 or names its sensor L. Because a phantom pressure would suggest an application that is not taking place, the module stops trusting the channel.

Likely causes

Sensor ground open, so the signal floats up to supply level
Signal line shorted to the 5 V reference or battery voltage
Sensor electronics driving the output to its upper limit

Symptoms

  • ABS/ESC warning lamp on
  • Stability control or electronic brake pressure control switched off

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–1,600
Time0.5–3 h

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

Parts involved

brake pressure sensor lsensor wiring

Related codes

C0645C0647C0648C064AC064EC0642

References: patents.google.com, wabco-customercentre.com.