C0040

Symptoms

• ABS warning light on
• traction/stability control disabled with possible 'Service StabiliTrak/ESC' message
• ABS non-functional
• speedometer
• cruise control or hill-hold features may behave erratically on some vehicles
• intermittent low-speed ABS activation or pulsation if the signal is noisy.

Probable causes

• 1) Failed right-front wheel speed sensor (internal open/short or contaminated encoder). 2) Harness or connector faults
• typically in the flexing section at the knuckle/fender-liner: broken wire
• corrosion or water intrusion in the connector
• loose or spread terminals
• tire-rub damage. 3) Tone ring/encoder damage (chipped teeth, rust, cracks) or wheel bearing play / improper sensor air gap. 4) Rarely
• an ABS module input circuit fault.

Diagnostic steps

1. Connect a scan tool, confirm C0040 and note any related codes; monitor live wheel-speed data and compare the right-front reading to the other three wheels while driving at steady low speed.
2. Visually inspect the right-front knuckle: sensor seating, debris, broken harness retainers, pigtail connector condition; unplug the connector and check pins for dirt, rust or bent contacts.
3. Test the sensor per its type: for active sensors verify supply voltage and ground; for passive sensors scope the AC waveform or measure sensor resistance against the service-manual spec.
4. Inspect the encoder/tone ring for rust, cracks or missing segments and check for wheel bearing play.
5. Repair or replace the failed component (sensor, wiring, tone ring/bearing), clear the code and retest to confirm the fix.
6. If all inputs check good, suspect the ABS module input circuit. Treat as high priority - foundation brakes still work but ABS/ESC support is reduced.