Code
C0121
Generic
C — Chassis
Left Rear Wheel Speed Sensor Circuit
Views:
UK: 2
EN: 1
RU: 1
AI status
Completed
Completed
100%
Causes
- Open or short in left rear wheel speed sensor wiring
- Corroded or loose connector at the sensor or ABS module
- Failed (shorted or open) wheel speed sensor
- Damaged or missing tone/reluctor ring on the hub or axle
- Excessive wheel bearing play or metal debris fouling the sensor
- Intermittent contact due to broken wire or chafing
Symptoms
- ABS warning lamp illuminated
- Traction control or stability control warning / disabled
- ABS/traction control interventions not available or working improperly
- Possible ABS pump or warning light activation during braking
- Erratic speed reading for that wheel on a scan tool; possible speedometer/ cruise control effects depending on vehicle
- Intermittent faults that may clear and return
What to check
- Scan tool: read freeze frame and live wheel speed data; compare left rear to other wheels at low speed
- Visual inspection of left rear sensor, connector, wiring harness, and tone ring/hub
- Backprobe sensor connector for reference voltage/signal (with ignition ON) using a multimeter or oscilloscope
- Measure sensor resistance and continuity to the ABS module with ignition OFF
- Check for AC/pulsed voltage output while spinning the wheel (passive sensors produce AC; active sensors produce a pulsed DC/square wave)
- Wiggle test wiring and connectors while monitoring live data to reveal intermittent opens/shorts
Signal parameters
- Passive (magnetic) sensor: produces alternating voltage whose amplitude and frequency increase with wheel speed; amplitude typically rises from near 0 V at idle to several hundred millivolts or volts at speed (varies by design)
- Active (Hall/VR with electronics) sensor: outputs a pulsed/square-wave signal referenced to system ground; common logic levels are 0–5 V or 0–12 V depending on vehicle
- Resistance check (passive sensor): often in the hundreds to a few thousand ohms range—specification varies by manufacturer
- Frequency of pulses is proportional to wheel rotational speed; an oscilloscope will show a clean consistent waveform if sensor and tone ring are good
- Reference or supply voltage (if present) is typically a regulated 5 V or 12 V from the ABS module—check vehicle-specific spec
Diagnostic algorithm
- Connect a scan tool and record freeze-frame data and live wheel speed readings; note if left rear differs from other wheels.
- Visual inspection: check the left rear sensor, plug, and harness for damage, corrosion, or contamination. Inspect tone ring for missing/damaged teeth and debris.
- With ignition OFF, disconnect the sensor connector and measure sensor resistance (if passive) and check for short to ground/power. Compare to factory specification where available.
- With sensor connected and vehicle safely lifted, spin the wheel and observe signal with an oscilloscope or multimeter (AC for passive; volts for active). Look for clean, consistent waveform or voltage pulses.
- Wiggle wiring and connector while monitoring live data to detect intermittent wiring faults.
- Backprobe the sensor connector to verify reference voltage and signal return at the module with ignition ON. Also check continuity between the sensor connector and ABS module pins.
- Repair found faults: clean/replace corroded connector, repair wiring harness, replace sensor, or replace damaged tone ring/hub as required.
- After repairs, clear codes, perform a road test while monitoring wheel speed signals to confirm fault does not return.
- If wiring and sensor check good but fault persists, consider testing/replacing the ABS module or taking vehicle to a specialist for module-level diagnostics.
Likely causes
- Damaged sensor or connector (most common)
- Wiring open/short between sensor and ABS module
- Contaminated or damaged tone ring on the hub
- Connector corrosion or water ingress causing intermittent contact
- Wheel bearing failure causing sensor misalignment
Fault status
Status
Left rear wheel speed sensor circuit fault. ABS module detected an open/short/intermittent or implausible signal from the left rear wheel speed sensor circuit. May disable ABS/traction control until fault is corrected.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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Code
C0121
HUMMER
C — Chassis
Steering Angle Sensor Circuit Range/Performance
Views:
UK: 14
EN: 13
RU: 12
AI status
Completed
Completed
100%
Causes
- Open or short in left rear wheel speed sensor wiring
- Corroded or loose connector at the sensor or ABS module
- Failed (shorted or open) wheel speed sensor
- Damaged or missing tone/reluctor ring on the hub or axle
- Excessive wheel bearing play or metal debris fouling the sensor
- Intermittent contact due to broken wire or chafing
Symptoms
- ABS warning lamp illuminated
- Traction control or stability control warning / disabled
- ABS/traction control interventions not available or working improperly
- Possible ABS pump or warning light activation during braking
- Erratic speed reading for that wheel on a scan tool; possible speedometer/ cruise control effects depending on vehicle
- Intermittent faults that may clear and return
What to check
- Scan tool: read freeze frame and live wheel speed data; compare left rear to other wheels at low speed
- Visual inspection of left rear sensor, connector, wiring harness, and tone ring/hub
- Backprobe sensor connector for reference voltage/signal (with ignition ON) using a multimeter or oscilloscope
- Measure sensor resistance and continuity to the ABS module with ignition OFF
- Check for AC/pulsed voltage output while spinning the wheel (passive sensors produce AC; active sensors produce a pulsed DC/square wave)
- Wiggle test wiring and connectors while monitoring live data to reveal intermittent opens/shorts
Signal parameters
- Passive (magnetic) sensor: produces alternating voltage whose amplitude and frequency increase with wheel speed; amplitude typically rises from near 0 V at idle to several hundred millivolts or volts at speed (varies by design)
- Active (Hall/VR with electronics) sensor: outputs a pulsed/square-wave signal referenced to system ground; common logic levels are 0–5 V or 0–12 V depending on vehicle
- Resistance check (passive sensor): often in the hundreds to a few thousand ohms range—specification varies by manufacturer
- Frequency of pulses is proportional to wheel rotational speed; an oscilloscope will show a clean consistent waveform if sensor and tone ring are good
- Reference or supply voltage (if present) is typically a regulated 5 V or 12 V from the ABS module—check vehicle-specific spec
Diagnostic algorithm
- Connect a scan tool and record freeze-frame data and live wheel speed readings; note if left rear differs from other wheels.
- Visual inspection: check the left rear sensor, plug, and harness for damage, corrosion, or contamination. Inspect tone ring for missing/damaged teeth and debris.
- With ignition OFF, disconnect the sensor connector and measure sensor resistance (if passive) and check for short to ground/power. Compare to factory specification where available.
- With sensor connected and vehicle safely lifted, spin the wheel and observe signal with an oscilloscope or multimeter (AC for passive; volts for active). Look for clean, consistent waveform or voltage pulses.
- Wiggle wiring and connector while monitoring live data to detect intermittent wiring faults.
- Backprobe the sensor connector to verify reference voltage and signal return at the module with ignition ON. Also check continuity between the sensor connector and ABS module pins.
- Repair found faults: clean/replace corroded connector, repair wiring harness, replace sensor, or replace damaged tone ring/hub as required.
- After repairs, clear codes, perform a road test while monitoring wheel speed signals to confirm fault does not return.
- If wiring and sensor check good but fault persists, consider testing/replacing the ABS module or taking vehicle to a specialist for module-level diagnostics.
Likely causes
- Damaged sensor or connector (most common)
- Wiring open/short between sensor and ABS module
- Contaminated or damaged tone ring on the hub
- Connector corrosion or water ingress causing intermittent contact
- Wheel bearing failure causing sensor misalignment
Fault status
Status
Left rear wheel speed sensor circuit fault. ABS module detected an open/short/intermittent or implausible signal from the left rear wheel speed sensor circuit. May disable ABS/traction control until fault is corrected.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
Similar codes
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Code
C0121
HYUNDAI
C — Chassis
Valve Relay Circuit Malfunction
Views:
UK: 17
EN: 22
RU: 19
AI status
Completed
Completed
100%
Causes
- Open or short in left rear wheel speed sensor wiring
- Corroded or loose connector at the sensor or ABS module
- Failed (shorted or open) wheel speed sensor
- Damaged or missing tone/reluctor ring on the hub or axle
- Excessive wheel bearing play or metal debris fouling the sensor
- Intermittent contact due to broken wire or chafing
Symptoms
- ABS warning lamp illuminated
- Traction control or stability control warning / disabled
- ABS/traction control interventions not available or working improperly
- Possible ABS pump or warning light activation during braking
- Erratic speed reading for that wheel on a scan tool; possible speedometer/ cruise control effects depending on vehicle
- Intermittent faults that may clear and return
What to check
- Scan tool: read freeze frame and live wheel speed data; compare left rear to other wheels at low speed
- Visual inspection of left rear sensor, connector, wiring harness, and tone ring/hub
- Backprobe sensor connector for reference voltage/signal (with ignition ON) using a multimeter or oscilloscope
- Measure sensor resistance and continuity to the ABS module with ignition OFF
- Check for AC/pulsed voltage output while spinning the wheel (passive sensors produce AC; active sensors produce a pulsed DC/square wave)
- Wiggle test wiring and connectors while monitoring live data to reveal intermittent opens/shorts
Signal parameters
- Passive (magnetic) sensor: produces alternating voltage whose amplitude and frequency increase with wheel speed; amplitude typically rises from near 0 V at idle to several hundred millivolts or volts at speed (varies by design)
- Active (Hall/VR with electronics) sensor: outputs a pulsed/square-wave signal referenced to system ground; common logic levels are 0–5 V or 0–12 V depending on vehicle
- Resistance check (passive sensor): often in the hundreds to a few thousand ohms range—specification varies by manufacturer
- Frequency of pulses is proportional to wheel rotational speed; an oscilloscope will show a clean consistent waveform if sensor and tone ring are good
- Reference or supply voltage (if present) is typically a regulated 5 V or 12 V from the ABS module—check vehicle-specific spec
Diagnostic algorithm
- Connect a scan tool and record freeze-frame data and live wheel speed readings; note if left rear differs from other wheels.
- Visual inspection: check the left rear sensor, plug, and harness for damage, corrosion, or contamination. Inspect tone ring for missing/damaged teeth and debris.
- With ignition OFF, disconnect the sensor connector and measure sensor resistance (if passive) and check for short to ground/power. Compare to factory specification where available.
- With sensor connected and vehicle safely lifted, spin the wheel and observe signal with an oscilloscope or multimeter (AC for passive; volts for active). Look for clean, consistent waveform or voltage pulses.
- Wiggle wiring and connector while monitoring live data to detect intermittent wiring faults.
- Backprobe the sensor connector to verify reference voltage and signal return at the module with ignition ON. Also check continuity between the sensor connector and ABS module pins.
- Repair found faults: clean/replace corroded connector, repair wiring harness, replace sensor, or replace damaged tone ring/hub as required.
- After repairs, clear codes, perform a road test while monitoring wheel speed signals to confirm fault does not return.
- If wiring and sensor check good but fault persists, consider testing/replacing the ABS module or taking vehicle to a specialist for module-level diagnostics.
Likely causes
- Damaged sensor or connector (most common)
- Wiring open/short between sensor and ABS module
- Contaminated or damaged tone ring on the hub
- Connector corrosion or water ingress causing intermittent contact
- Wheel bearing failure causing sensor misalignment
Fault status
Status
Left rear wheel speed sensor circuit fault. ABS module detected an open/short/intermittent or implausible signal from the left rear wheel speed sensor circuit. May disable ABS/traction control until fault is corrected.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
Similar codes
Your experience will help others
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Code
C0121
OPEL
C — Chassis
Valve Relay Circuit Malfunction
Views:
UK: 5
EN: 7
RU: 3
AI status
Completed
Completed
100%
Causes
- Open or short in left rear wheel speed sensor wiring
- Corroded or loose connector at the sensor or ABS module
- Failed (shorted or open) wheel speed sensor
- Damaged or missing tone/reluctor ring on the hub or axle
- Excessive wheel bearing play or metal debris fouling the sensor
- Intermittent contact due to broken wire or chafing
Symptoms
- ABS warning lamp illuminated
- Traction control or stability control warning / disabled
- ABS/traction control interventions not available or working improperly
- Possible ABS pump or warning light activation during braking
- Erratic speed reading for that wheel on a scan tool; possible speedometer/ cruise control effects depending on vehicle
- Intermittent faults that may clear and return
What to check
- Scan tool: read freeze frame and live wheel speed data; compare left rear to other wheels at low speed
- Visual inspection of left rear sensor, connector, wiring harness, and tone ring/hub
- Backprobe sensor connector for reference voltage/signal (with ignition ON) using a multimeter or oscilloscope
- Measure sensor resistance and continuity to the ABS module with ignition OFF
- Check for AC/pulsed voltage output while spinning the wheel (passive sensors produce AC; active sensors produce a pulsed DC/square wave)
- Wiggle test wiring and connectors while monitoring live data to reveal intermittent opens/shorts
Signal parameters
- Passive (magnetic) sensor: produces alternating voltage whose amplitude and frequency increase with wheel speed; amplitude typically rises from near 0 V at idle to several hundred millivolts or volts at speed (varies by design)
- Active (Hall/VR with electronics) sensor: outputs a pulsed/square-wave signal referenced to system ground; common logic levels are 0–5 V or 0–12 V depending on vehicle
- Resistance check (passive sensor): often in the hundreds to a few thousand ohms range—specification varies by manufacturer
- Frequency of pulses is proportional to wheel rotational speed; an oscilloscope will show a clean consistent waveform if sensor and tone ring are good
- Reference or supply voltage (if present) is typically a regulated 5 V or 12 V from the ABS module—check vehicle-specific spec
Diagnostic algorithm
- Connect a scan tool and record freeze-frame data and live wheel speed readings; note if left rear differs from other wheels.
- Visual inspection: check the left rear sensor, plug, and harness for damage, corrosion, or contamination. Inspect tone ring for missing/damaged teeth and debris.
- With ignition OFF, disconnect the sensor connector and measure sensor resistance (if passive) and check for short to ground/power. Compare to factory specification where available.
- With sensor connected and vehicle safely lifted, spin the wheel and observe signal with an oscilloscope or multimeter (AC for passive; volts for active). Look for clean, consistent waveform or voltage pulses.
- Wiggle wiring and connector while monitoring live data to detect intermittent wiring faults.
- Backprobe the sensor connector to verify reference voltage and signal return at the module with ignition ON. Also check continuity between the sensor connector and ABS module pins.
- Repair found faults: clean/replace corroded connector, repair wiring harness, replace sensor, or replace damaged tone ring/hub as required.
- After repairs, clear codes, perform a road test while monitoring wheel speed signals to confirm fault does not return.
- If wiring and sensor check good but fault persists, consider testing/replacing the ABS module or taking vehicle to a specialist for module-level diagnostics.
Likely causes
- Damaged sensor or connector (most common)
- Wiring open/short between sensor and ABS module
- Contaminated or damaged tone ring on the hub
- Connector corrosion or water ingress causing intermittent contact
- Wheel bearing failure causing sensor misalignment
Fault status
Status
Left rear wheel speed sensor circuit fault. ABS module detected an open/short/intermittent or implausible signal from the left rear wheel speed sensor circuit. May disable ABS/traction control until fault is corrected.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
Similar codes
Your experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
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