Code
C0039
ALFA ROMEO
C — Chassis
Right rear tone wheel / speed sensor fault
Views:
UK: 4
EN: 9
RU: 5
AI status
Completed
Completed
100%
Causes
- Damaged or disconnected wheel speed sensor (right rear)
- Corroded, loose or damaged connector or wiring harness to the sensor
- Broken, cracked or missing teeth on the tone wheel (reluctor ring)
- Incorrect air gap between sensor and tone wheel (misalignment)
- Short to battery or ground in the sensor circuit
- Contaminated sensor (metal debris, brake dust) or physical damage
Symptoms
- ABS, ESP/traction control warning lamp illuminated
- Loss of anti-lock braking or reduced traction control functionality
- Inconsistent or no wheel speed reading for the right rear wheel on scan tool
- Intermittent faults depending on wheel position or road conditions
- Possible unusual brake feel during ABS activation (rare)
What to check
- Read ABS/ESP module trouble codes and freeze frame data; note any related communications codes
- Compare live wheel speed sensor data for all four wheels at low speed
- Visually inspect the right rear sensor, connector and harness for damage, corrosion or contamination
- Inspect tone wheel (reluctor) for cracks, missing teeth, rust or debris
- Wiggle test wiring while monitoring live data to reproduce intermittent faults
- Backprobing: check sensor supply (if active/Hall) and signal, and measure resistance for passive sensors
Signal parameters
- Passive (variable reluctance) sensor: AC voltage amplitude increases with wheel speed; check with shaft rotation
- Active (Hall/effect) sensor: DC reference (usually 5V) and square wave output (0–5V) or open collector pulsed signal
- Signal frequency/pulse count proportional to wheel speed; waveform should match other wheels in timing and shape
- Typical sensor resistance: consult OEM data (passive sensors often hundreds to a few thousand ohms); use manufacturer specs
- Expected air gap: small and consistent (check OEM spec, commonly 0.5–2.0 mm)
Diagnostic algorithm
- Retrieve full code set, note freeze frame and any other related codes (CAN/communication or other wheel sensors).
- With scan tool, monitor live wheel speed values while rotating the vehicle at low speed or lifting the wheel; confirm right rear shows no or erratic signal.
- Visually inspect right rear sensor, tone wheel and harness for damage, contamination or loose mounting. Clean if contaminated.
- Verify connector integrity: disconnect, inspect pins for corrosion/bent pins, clean and apply dielectric grease if appropriate.
- Measure sensor circuit: for passive sensor measure AC voltage while rotating wheel; for active sensor check reference voltage and signal waveform with oscilloscope or digital multimeter.
- Perform continuity and short tests on wiring: check for open, short-to-ground and short-to-battery between sensor pins and ABS module connector.
- Check air gap and sensor mounting alignment; correct gap or reseat sensor per OEM spec if out of tolerance.
- If wiring and sensor tests fail, replace the sensor and retest. If new sensor still fails and wiring is confirmed good, evaluate ABS module input and grounds.
- After repair, clear codes and perform road test to confirm signal stability and that the warning lamps do not return.
Likely causes
- Wiring/connector corrosion or open circuit at the right rear sensor
- Sensor damaged during service or by road debris
- Tone wheel damaged (impact/corrosion) or missing tooth(s)
- Intermittent connection from chafed wiring or moisture ingress
- Module input fault (less common) after wiring and sensor checks
Fault status
Status
Right rear wheel speed sensor/tone wheel signal missing, out of range, or intermittent. ABS/ESP module flag for right rear wheel speed input.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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