Chassis, generic

C0649 Brake Pressure Sensor M Circuit/Open

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

The circuit of brake pressure sensor M, the twelfth in the series, has no continuity. Where so many pressure inputs exist, they are often not reachable from outside: WABCO notes that the two rear axle pressure sensors of its truck EBS are wired inside the axle modulator, so the cabling cannot be tested from outside and a fault means work on the modulator. Passenger-car units behave similarly; Actronics points out that the ATE MK60 carries its brake pressure sensor inside the unit, while some VW models use separate sensors near the master cylinder. External sensors, as on WABCO's trailer brake, take their supply from the modulator through a plug. No carmaker assigns C0649 or the letter M. Check plug and terminals first, then the unit.

Likely causes

Loose or corroded plug at an external sensor or at the modulator
Open in the internal wiring of the modulator or hydraulic unit
Broken wire in the sensor harness

Symptoms

  • Brake or ABS warning lamp on
  • Electronic pressure control replaced by a fallback mode

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–2,000
Time0.5–4 h

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

Parts involved

brake pressure sensor msensor wiring

Related codes

C064AC064BC064CC0645C064DC0641

References: wabco-customercentre.com, wabco-customercentre.com, actronics.co.uk.