ABS and brakes

C0040 — Right Front Wheel Speed Sensor Circuit Fault

Severity: medium Drive with care, to a shop

What C0040 means

The ABS control module constantly compares the signals from each wheel speed sensor. C0040 means the signal or circuit for the right front sensor is missing, erratic, or out of range. The sensor reads a toothed tone ring on the hub or axle, and without that signal ABS and stability control cannot work properly.

Symptoms

  • ABS warning light on
  • Traction or stability control light on
  • ABS does not activate in hard braking
  • Traction control disabled
  • Erratic speedometer reading on some vehicles

Possible causes

  1. Failed right front wheel speed sensor
  2. Damaged or corroded wiring and connectors in the sensor circuit
  3. Damaged, dirty, or cracked tone ring (reluctor)
  4. Brake dust or metal debris on the sensor tip, or wrong air gap
  5. Chafed wiring harness near the strut and wheel well
  6. Worn wheel bearing causing tone ring runout
  7. Faulty ABS control module (uncommon)

How to diagnose

  1. Scan the ABS module for codes, review freeze frame data, and note any codes for other wheels
  2. Compare live wheel speed data for all four wheels on a test drive or with the wheels lifted
  3. Inspect the sensor, its mounting, and the harness from the wheel to the body for chafing, breaks, and corrosion
  4. Unplug the connector, check the pins for corrosion or moisture, and test the circuit for opens and shorts to ground
  5. Remove the sensor, clean the tip, and look for damage
  6. Inspect the tone ring for cracks, rust, or clogged teeth, and check the wheel bearing for play
  7. Test the sensor resistance or signal with a scope per the manufacturer's specs
  8. If the sensor and wiring are good, check power and ground at the ABS module

Common fixes

  • Replace the right front wheel speed sensor
  • Repair the wiring or replace the damaged harness or connector
  • Clean or replace the tone ring, hub, or wheel bearing
  • Replace or repair the ABS control module, with programming if needed

Questions and answers

Can I drive with a C0040 code?

Yes, but carefully. The regular brakes usually still work, while ABS and stability control are off. Leave extra following distance and get it diagnosed soon.

What most often causes C0040?

Most often it is the wheel speed sensor itself or its wiring, which gets damaged near the hub and strut. A dirty sensor or a damaged tone ring can also cause it.

Can I just clear the code?

You can clear it, but it will return if the cause is still there. Find and fix the problem in the sensor, wiring, or tone ring first.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.