Chassis, generic

C0644 Brake Pressure Sensor K Range/Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

The voltage from sensor K is in range, yet the pressure is not believable. Bendix's EC-80 stores a plausibility error when the primary and secondary brake demand sensors disagree and asks for a static calibration after an ECU swap. Mercedes' SBC repeats self-tests about every 16 brake applications, pressurising the calipers to check sensors, valves and leaks; if the system fails, two valves stay open so pedal force acts on the brakes directly. A Bosch patent keeps an offset fault stored, with ABS and ESC lamps lit, until a later good check on a roughly straight drive passes. No manufacturer documents C0644 or says which sensor K is. Look for drift, air, a leak or a valve that does not close.

Likely causes

Sensor K offset or gain drifted
Sensor values not re-learned after module replacement
Leak, air or a valve that does not seal distorting the pressure

Symptoms

  • ABS and ESC lamps on until a later successful check
  • Stability control reduced, pedal may feel different in fallback mode

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–2,000
Time0.5–4 h

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

Parts involved

brake pressure sensor k

Related codes

C0641C0642C0643C0640C0634

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