ABS and brakes

C0039 — Right Rear Wheel Speed Sensor Circuit Malfunction

Severity: medium Drive with care, to a shop

What C0039 means

The ABS module constantly reads the speed signal from each wheel and compares them. With C0039, the signal or circuit of the right rear wheel speed sensor is missing, erratic or out of range compared to the other wheels. Without this data, ABS and stability control cannot tell whether a wheel is locking up or spinning.

Symptoms

  • ABS warning light on
  • Traction and stability control disabled, with their light possibly on
  • False ABS activation during braking
  • Erratic or non-working speedometer on some models
  • Brake pedal pulsing at low speed

Possible causes

  1. Failed right rear wheel speed sensor
  2. Damaged, dirty or rusty tone (exciter) ring
  3. Broken, chafed or damaged wiring in the wheel well harness
  4. Corrosion or poor contact at the sensor connector
  5. Metal debris or dirt on the sensor tip
  6. Wrong air gap after hub or bearing work

How to diagnose

  1. Scan for codes and confirm C0039 returns after clearing
  2. Inspect the sensor and harness at the right rear wheel for breaks, chafing and past repairs
  3. Check the connector for corrosion, moisture and a secure fit
  4. Remove the sensor, clean off debris and check its seating and air gap
  5. Inspect the tone ring for cracks, missing teeth and rust
  6. Test sensor resistance and wire continuity to the ABS module, plus shorts to ground or power
  7. Compare wheel speeds in live data on a test drive; the right rear should match the others

Common fixes

  • Clean the sensor and connector and apply contact grease
  • Replace the right rear wheel speed sensor
  • Repair or replace the damaged wiring section
  • Replace the hub or bearing with the damaged tone ring
  • Clear the code and confirm the ABS light stays off

Questions and answers

Can I drive with code C0039?

Yes, but carefully and not for long. The standard brakes work, but ABS and stability control are likely off. This is especially risky on wet or slippery roads.

What most often causes C0039?

Most often it is the sensor itself or its wiring in the wheel well: a break, a corroded connector or dirt. A damaged tone ring is also possible.

Why is the ABS light still on after replacing the sensor?

The code must be cleared with a scan tool. Also check the connector, sensor air gap and tone ring condition, as the sensor may not have been the only problem.

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.