Damaged or corroded wheel speed sensor connector at the left rear wheel, often from water intrusion or road debris; Broken, chafed, or stretched sensor wiring harness near the left rear suspension or wheel well; Failed left rear wheel speed sensor due to age, impact, or metallic debris stuck to the sensor tip; Damaged or dirty sensor tone ring (reluctor ring) on the left rear hub or axle, including cracks or missing teeth; Excessive wheel bearing play or hub runout at the left rear wheel disturbing the sensor air gap; Corrosion or terminal damage at the ABS module connector for the left rear sensor circuit; Internal fault in the ABS control module affecting only the left rear sensor input channel

DTC codes / C00xx · Brakes, steering and suspension / C0040

C0040Left Rear Wheel Speed Sensor Circuit

Q: What does code C0040 mean?

Left Rear Wheel Speed Sensor Circuit. C0040 means the antilock brake system (ABS) module has detected an abnormal signal from the left rear wheel speed sensor circuit, which can include an open circuit, short to ground or power, or an erratic signal. The computer compares that signal with the other three wheel speed sensors and sets this code when the left rear reading is missing, implausible, or inconsistent.

C0040 means the antilock brake system (ABS) module has detected an abnormal signal from the left rear wheel speed sensor circuit, which can include an open circuit, short to ground or power, or an erratic signal. The computer compares that signal with the other three wheel speed sensors and sets this code when the left rear reading is missing, implausible, or inconsistent.

SystemChassis

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaBrakes, steering and suspension

Symptoms you may notice

ABS warning light illuminated on the dashboard, often with the traction control and stability control lights also on
Often no symptoms besides the warning light, with normal braking performance in dry conditions
ABS may deactivate, allowing wheel lockup during hard braking on slippery surfaces
Traction control or stability control may stop working, reducing grip on low-friction roads
Speedometer may read erratically or drop out if the module uses that sensor for vehicle speed
A pulsation or unusual pedal feel during low-speed stops if the sensor signal is intermittent

Common causes, most likely first

Damaged or corroded wheel speed sensor connector at the left rear wheel, often from water intrusion or road debris
Broken, chafed, or stretched sensor wiring harness near the left rear suspension or wheel well
Failed left rear wheel speed sensor due to age, impact, or metallic debris stuck to the sensor tip
Damaged or dirty sensor tone ring (reluctor ring) on the left rear hub or axle, including cracks or missing teeth
Excessive wheel bearing play or hub runout at the left rear wheel disturbing the sensor air gap
Corrosion or terminal damage at the ABS module connector for the left rear sensor circuit
Internal fault in the ABS control module affecting only the left rear sensor input channel

How to diagnose and fix it

Inspect the left rear wheel speed sensor, its connector, and the wiring harness for physical damage, corrosion, or loose pins
Clean the sensor tip and tone ring, then check the tone ring for cracks, missing teeth, or heavy rust
Measure the sensor circuit resistance and compare it with the right rear sensor or the manufacturer's specification
Back-probe the sensor connector and check for an alternating current (AC) voltage signal while spinning the left rear wheel by hand
Check the sensor air gap and inspect the left rear wheel bearing and hub for excessive play or runout
Verify continuity and isolation of both sensor wires from the sensor connector back to the ABS module connector
If all wiring and sensor checks pass, test or replace the ABS control module only after confirming its ground and power feeds

What a left rear speed sensor code hides

A dirty or rusted tone ring often sets C0040 even when the sensor itself tests good, so always inspect the ring before condemning the sensor.
Intermittent codes frequently come from a harness that flexes with suspension travel, so wiggle-test the wiring while watching live wheel speed data.
A failing left rear wheel bearing can alter the sensor air gap and trigger this code without any obvious noise, so check hub play during diagnosis.
Some vehicles share the left rear wheel speed signal with the speedometer or transmission controller, so a faulty sensor can cause drivability symptoms beyond the ABS light.
Replacing the sensor without clearing and rechecking live data can leave the code stored if the real fault is a corroded connector or module terminal.

Can I keep driving with C0040?

You can usually drive the vehicle with C0040 because normal braking remains, but the ABS and stability control systems may be disabled, so avoid hard braking and slippery conditions until the fault is repaired.

Tools that help

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