TR
EN
C0039 Right Rear ABS Sensor Ring Malfunction
C0039 Right Rear ABS Sensor Ring Malfunction is a diagnostic trouble code that indicates the vehicle's Anti-lock Braking System (ABS) Control Module has detected a malfunction with the right rear ABS sensor ring, also known as the tone ring, reluctor ring, or encoder ring.
The ABS sensor ring is attached to the wheel hub, axle shaft, CV joint, or wheel bearing, depending on the vehicle design. As the wheel rotates, the ABS wheel speed sensor reads the teeth or magnetic encoder built into the sensor ring to determine the wheel's rotational speed.
If the ABS control module detects missing, distorted, or inconsistent signals caused by a damaged sensor ring, it stores the C0039 diagnostic trouble code and usually illuminates the ABS warning light.
This fault is commonly caused by a cracked reluctor ring, rust damage, missing teeth, excessive dirt, wheel bearing failure, incorrect sensor gap, or physical damage.
What Does C0039 Right Rear ABS Sensor Ring Malfunction Mean?
The right rear ABS sensor ring generates the signal used by the wheel speed sensor.
As the wheel rotates, the sensor ring passes in front of the wheel speed sensor, producing an electrical signal that allows the ABS module to calculate wheel speed.
The ABS module continuously monitors:
-
Wheel speed signal
-
Signal frequency
-
Signal consistency
-
Wheel acceleration
-
Wheel deceleration
If the sensor ring becomes damaged or distorted, the signal becomes irregular and the ABS module stores C0039.
Possible causes include:
-
Cracked ABS sensor ring
-
Missing reluctor ring teeth
-
Rust expansion
-
Magnetic encoder damage
-
Wheel bearing failure
-
Excessive contamination
-
Improper sensor gap
What Does the ABS Sensor Ring Do?
The ABS sensor ring provides the reference pattern that the wheel speed sensor uses to calculate wheel speed.
The information is required for:
-
Anti-lock Braking System (ABS)
-
Electronic Stability Control (ESC)
-
Traction Control System (TCS)
-
Hill Start Assist
-
Trailer Stability Control
-
Adaptive Cruise Control (on some vehicles)
Without a properly functioning sensor ring, accurate wheel speed measurement is impossible.
Why Is the ABS Sensor Ring Important?
The ABS module compares the speed of all four wheels.
If one wheel suddenly slows faster than the others during braking, the module reduces brake pressure to prevent wheel lockup.
A damaged sensor ring produces inaccurate wheel speed information, causing the ABS system to lose proper control.
Symptoms of C0039
Symptoms vary depending on the severity of the fault.
ABS Warning Light
The ABS warning light is the most common symptom.
Brake Warning Light
Some vehicles also illuminate the brake warning light.
Stability Control Warning
Electronic Stability Control (ESC) may become disabled.
Traction Control Disabled
Traction Control System operation may be unavailable.
Cruise Control Problems
Some vehicles disable cruise control when wheel speed information becomes unreliable.
Pulsating Brake Pedal at Low Speed
A damaged sensor ring may cause the ABS system to activate unnecessarily during low-speed braking.
No Noticeable Braking Problems
Normal hydraulic braking usually continues to operate, but ABS protection may be unavailable.
Common Causes of C0039
Several mechanical problems may trigger this code.
Cracked ABS Sensor Ring
The reluctor ring may crack because of:
-
Metal fatigue
-
Corrosion
-
Impact damage
Even a small crack can distort the wheel speed signal.
Rust Expansion
Steel reluctor rings commonly expand because of rust.
This expansion may:
-
Change tooth spacing
-
Contact the sensor
-
Distort the signal
Missing or Damaged Teeth
The sensor ring may develop:
-
Broken teeth
-
Worn teeth
-
Bent teeth
The wheel speed sensor cannot accurately read a damaged reluctor pattern.
Magnetic Encoder Damage
Many modern wheel bearings use a magnetic encoder instead of a traditional toothed reluctor ring.
Possible failures include:
-
Demagnetization
-
Physical damage
-
Contamination
Wheel Bearing Failure
A worn wheel bearing may cause:
-
Excessive movement
-
Incorrect sensor gap
-
Encoder damage
Dirt or Metal Debris
Excessive contamination around the sensor ring may interfere with signal generation.
Examples include:
-
Mud
-
Rust
-
Metal filings
-
Grease contamination
Incorrect Sensor Gap
Improper clearance between the sensor and reluctor ring may produce weak or distorted signals.
How Is C0039 Diagnosed?
Proper diagnosis requires inspection of both the sensor ring and the wheel speed sensor.
Scan the ABS Module
A professional diagnostic scanner should retrieve:
-
ABS fault codes
-
Live wheel speed data
-
Freeze-frame information
Related ABS codes often provide additional diagnostic clues.
Monitor Live Wheel Speed Data
Technicians compare:
-
Left rear wheel speed
-
Right rear wheel speed
-
Front wheel speeds
A damaged sensor ring often causes erratic or intermittent readings.
Inspect the ABS Sensor Ring
The sensor ring should be inspected for:
-
Cracks
-
Missing teeth
-
Rust damage
-
Dirt buildup
-
Physical deformation
Inspect the Wheel Speed Sensor
The sensor should be checked for:
-
Proper mounting
-
Damage
-
Metal contamination
-
Correct air gap
Inspect the Wheel Bearing
Technicians verify:
-
Bearing play
-
Bearing noise
-
Encoder condition
Inspect Wiring and Connectors
Electrical inspection should include:
-
Wiring continuity
-
Connector condition
-
Corrosion
-
Secure connections
How Is C0039 Fixed?
The repair depends on the actual cause.
Replace the ABS Sensor Ring
If the reluctor ring is cracked, rusted, or damaged, replacement is required.
Replace the Wheel Bearing
If the magnetic encoder is integrated into the wheel bearing, the complete bearing assembly must usually be replaced.
Replace the Wheel Speed Sensor
If the sensor has also been damaged, replacement may be necessary.
Repair Wiring Problems
Possible repairs include:
-
Repair damaged wires
-
Replace damaged wiring
-
Repair connectors
Clean the Sensor Ring
Removing:
-
Dirt
-
Rust
-
Metal particles
may restore proper signal quality.
Correct Sensor Gap
Improper sensor clearance should be adjusted according to manufacturer specifications.
After repairs:
-
Clear diagnostic trouble codes.
-
Monitor live wheel speed data.
-
Road test the vehicle.
-
Confirm that C0039 does not return.
Can You Drive With C0039?
The vehicle usually remains drivable because conventional hydraulic braking continues to function.
However, the following systems may become unavailable:
-
ABS
-
Electronic Stability Control
-
Traction Control
-
Brake Assist
Extra caution should be used on slippery road surfaces.
Is C0039 a Serious Code?
C0039 is generally considered a moderate safety-related fault.
Although normal braking usually remains available, the loss of ABS and stability control can significantly reduce vehicle safety during emergency braking or poor road conditions.
Prompt diagnosis and repair are recommended.
How to Prevent C0039
Regular maintenance helps reduce the likelihood of ABS sensor ring failures.
Inspect Wheel Bearings
Replace worn bearings before encoder damage occurs.
Keep the Sensor Area Clean
Regular cleaning helps prevent:
-
Rust buildup
-
Dirt accumulation
-
Metal contamination
Inspect Suspension Components
Excessive wheel movement can damage the sensor ring.
Address ABS Warning Lights Promptly
Early diagnosis often prevents additional brake system damage.
Final Thoughts
C0039 Right Rear ABS Sensor Ring Malfunction indicates that the ABS control module has detected a problem with the right rear reluctor ring or magnetic encoder used to generate the wheel speed signal.
The most common causes include:
-
Cracked ABS sensor ring
-
Rust damage
-
Missing reluctor teeth
-
Magnetic encoder failure
-
Wheel bearing problems
-
Excessive contamination
-
Incorrect sensor gap
A complete inspection of the right rear ABS sensor ring, wheel speed sensor, wheel bearing, wiring, connectors, and ABS control module is necessary to restore accurate wheel speed measurement and ensure proper operation of the ABS, traction control, and stability control systems.
