Code
C0037
ALFA ROMEO
C — Chassis
Left rear wheel speed sensor
Views:
UK: 3
EN: 20
RU: 15
AI status
Completed
Completed
100%
Causes
- Failed or damaged left rear wheel speed sensor
- Open or shorted wiring between sensor and ABS module
- Corroded or loose connector at sensor or harness
- Damaged or missing tone (reluctor) ring on hub/axle
- Excessive air gap or misalignment between sensor and tone ring
- Contamination (metallic debris, grease) on sensor/reluctor
Symptoms
- ABS warning lamp illuminated
- Traction control/stability control faults or disabled
- ABS braking performance degraded or altered
- Inconsistent or missing wheel speed reading for left rear in scan tool
- Brake/ABS pulsation or unusual noises from rear wheel
- Possible speedometer/odometer anomalies (less common)
What to check
- Scan ABS module for current/frozen DTCs and freeze frame data
- Visual inspection of left rear sensor, connector, and wiring for damage or corrosion
- Check for loose or missing tone/reluctor ring teeth and inspect wheel bearing
- Backprobe connector and measure sensor resistance (if applicable) and supply voltage
- Monitor wheel speed sensor channel with scan tool while rotating wheel (or on road test)
- If available, view sensor waveform with oscilloscope while wheel is rotated
Signal parameters
- Type: either passive (VR) or active (hall/magnetoresistive) depending on vehicle/version
- Passive (VR): DC resistance typically ~500–2,000 Ω (manufacturer-specific); produces AC voltage increasing with wheel speed (tens to hundreds of mV to volts depending on speed)
- Active (hall/variable): square or pulse waveform, 0–5 V (or 0–12 V) switching; frequency proportional to wheel speed
- Expected behaviour: clean, regular pulses/waveform with amplitude/frequency increasing with wheel rotation speed
- Abnormal: zero output, flatline, noisy or intermittent waveform, or constant voltage
Diagnostic algorithm
- Retrieve all stored codes and freeze-frame data using a compatible scan tool.
- Perform visual inspection of left rear sensor, harness, and connector for damage, contamination or corrosion; repair obvious damage.
- With ignition ON, backprobe connector: verify reference power/ground (if active sensor) or measure sensor resistance (if passive). Compare to OEM spec.
- Rotate wheel by hand or lift vehicle and road test while monitoring live wheel speed data; note if left rear reading is missing or erratic.
- Use a scope or multimeter (AC volts for VR sensor) to view waveform while wheel is rotated; look for clean periodic signal.
- Wiggle test wiring and connectors while monitoring data to identify intermittent opens/shorts.
- Inspect tone ring and wheel bearing for damage, missing teeth, or excessive play; correct as required.
- If wiring and tone ring are good, swap left rear sensor with another known-good sensor (if practical) to confirm sensor vs harness/module fault.
- Check ABS module power and ground circuits if signal and wiring appear good.
- After repair or replacement, clear codes and perform road test to verify the fault does not return.
Likely causes
- Wiring/connector fault at left rear sensor (most common)
- Failed left rear sensor element
- Damaged or dirty tone/reluctor ring at left rear wheel
- Incorrect sensor gap or misalignment
Fault status
Status
ABS/ESC control module has logged a fault for the left rear wheel speed sensor circuit; ABS warning lamp may be lit. Fault can be stored as current or intermittent — diagnostic data (freeze frame and live data) should be checked before repair.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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