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DTC codes / C02xx · Brakes, steering and suspension / C0237

C0237Loss of Speed Sensor Signal Front Left

C0237 means the antilock braking system (ABS) control module has lost the electrical signal from the front-left wheel speed sensor while the vehicle was moving. The module expects a steady alternating-current (AC) pulse train from that sensor, and it has seen the signal drop out, flatline, or become implausible compared with the other three wheels.

SystemChassis
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaBrakes, steering and suspension

Symptoms you may notice

  • The ABS warning light is illuminated steadily, and on many vehicles the traction control and stability control lights come on with it.
  • Often no symptoms besides the warning light, because the base brakes still stop the car normally.
  • The ABS may pulse or release unexpectedly at low speed as the module tries to work with a missing wheel signal.
  • Traction control or stability control may cut engine power or apply individual brakes without warning during normal driving.
  • The speedometer may fluctuate or read zero if the module shares the front-left signal with the instrument cluster.
  • A diagnostic trouble code (DTC) for an implausible or missing wheel speed signal is stored, and the ABS may disable itself until the next key cycle.

Common causes, most likely first

  1. A damaged or corroded front-left wheel speed sensor connector, often from road splash or a broken retaining clip.
  2. A chafed, pinched, or broken sensor harness near the strut, steering knuckle, or wheel well where it flexes with suspension travel.
  3. Metal debris, rust, or a damaged tone ring on the hub or CV joint that the sensor reads, causing a weak or erratic pulse.
  4. Excessive air gap between the sensor tip and the tone ring from a worn hub bearing, a bent knuckle, or a poorly seated sensor.
  5. An internally failing wheel speed sensor that works cold but drops out when hot or wet.
  6. A faulty ABS control module input circuit, though this is far less common than a sensor or wiring fault.
  7. A worn front-left wheel bearing that lets the hub move and disturbs the sensor-to-ring gap.

How to diagnose and fix it

  1. Scan all modules and record every stored and pending code, then clear codes and test-drive to confirm C0237 returns before touching parts.
  2. Inspect the front-left wheel speed sensor connector for corrosion, pushed pins, or a broken lock tab, and clean or repair as needed.
  3. Visually trace the sensor harness from the connector to the ABS module, looking for chafing, pinch points, or green corrosion in the wire.
  4. Measure the sensor's AC voltage output while spinning the wheel by hand; a healthy sensor produces a smooth rising and falling voltage.
  5. Check the tone ring through the sensor hole or by removing the sensor for rust, cracks, missing teeth, or metal shavings, and clean or replace it.
  6. Compare live wheel speed data from all four wheels on a road test to see whether the front-left signal drops out, spikes, or reads low.
  7. If the sensor, ring, and wiring all test good, perform a resistance and continuity check back to the ABS module before condemning the module.

What a wheel speed sensor code hides

  • A rusty or swollen tone ring is a frequent culprit that gets missed because the sensor itself tests fine on the bench; always inspect the ring the sensor reads.
  • A worn front-left wheel bearing can set C0237 without any growling noise yet, because the play changes the air gap and drops the signal at speed.
  • The ABS module often shares the front-left speed signal with the speedometer and transmission controller, so a bad sensor can cause shift or speedometer complaints that look unrelated.
  • Water intrusion into the connector after a car wash or deep puddle can set this code intermittently, and it may not return until the next wet drive.
  • Replacing the sensor without checking the harness flex point at the strut is a common misdiagnosis, because the wire often breaks inside intact-looking insulation.

Can I keep driving with C0237?

The car will still stop with normal hydraulic brakes, but ABS and stability control are disabled, so avoid hard braking, loose surfaces, and bad weather until the fault is repaired. Do not ignore the light if the brakes feel different or the pedal pulsates unexpectedly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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