C0040 on GMC: How to Diagnose & Fix
Right Front Wheel Speed Sensor Circuit
4 GMC models · 4 documented issues · 1 critical · $180-$900 repair
C0040 on GMC by model: symptoms, diagnosis and fix
The Lambda-platform hub bearing assemblies wear early — owners report failures well under 60,000 miles and, on the rear, repeat failures on the same vehicle. Two distinct symptoms come from one part: mechanical noise (a speed-dependent hum or growl that changes when the vehicle is loaded in a turn) and an ABS fault, because the encoder/tone ring on GM Lambda hubs is magnetically printed onto the bearing's inner race surface. When that magnetic layer degrades or sloughs off, the ABS module gets an implausible wheel speed signal and posts a wheel speed sensor code even though the sensor itself is fine — which is why replacing the sensor does not fix it.
Common Symptoms
- Humming or growling that rises with vehicle speed
- Noise changes when swerving left/right
- ABS and traction control warning lights
- Speedometer drop-out or erratic reading
- Play at the wheel when rocked at 12 and 6 o'clock
How to Fix
Diagnose by wheel with a chassis ear or stethoscope at each hub with the vehicle raised and unloaded, and confirm with a scan-tool comparison of the four wheel-speed signals at low speed — a hub with a damaged tone ring drops out or reads erratically while the others track. Replace the complete hub/bearing assembly (the bearing is not serviceable separately). Expect the axle shaft splines to be seized in the hub: plan on pressing them apart. Note that some aftermarket hubs ship without the ABS sensor, so buy the correct part number for your ABS configuration and torque the axle nut to spec.
Sources for this issue (4)
Front wheel hub bearing assemblies on Sierra 1500 trucks commonly wear out, especially on 4WD models and trucks used with larger tires or in rough conditions. Owners report humming or growling that changes with speed, looseness in the front wheel, and in some cases ABS warnings or low-speed ABS activation because the wheel speed sensor is integrated into the hub. This is a frequent repair item across both GMT800 and GMT900 generations.
Common Symptoms
- humming or growling from front end
- noise increases with vehicle speed
- wheel play or looseness
- ABS light on
- traction control warning
- low-speed ABS activation
How to Fix
Diagnosis involves checking for wheel play with the tire off the ground, listening for bearing noise during road tests, and comparing wheel speed sensor data if ABS symptoms are present. Repair is replacement of the affected hub assembly, and many owners replace both sides if mileage is high. Corrosion at the sensor mounting surfaces and wiring damage should also be addressed to prevent repeat ABS issues.
Sources for this issue (1)
GMT800 SUVs commonly develop corrosion under the front wheel speed sensors or wear in the hub bearing assembly, which alters the low-speed wheel speed signal. The result is unexpected ABS activation at parking-lot speeds, longer stopping distances, and a pulsating brake pedal even on dry pavement. GM documented this condition in service bulletins and in safety recall 05V379, which covers 2000-2002 Yukon XL in salt-belt states, and owners frequently report replacing hub assemblies to cure it.
Common Symptoms
- ABS activates at 3-10 mph on dry roads
- brake pedal kicks back at low speed
- longer stopping distance near a stop sign
- ABS warning light
- grinding or humming from front wheel bearing
How to Fix
Scan ABS wheel speed data during a low-speed stop and compare left and right front sensor readings. If one sensor drops out, inspect for rust buildup under the sensor or excessive hub play; cleaning the sensor mounting surface may help temporarily, but many vehicles need a complete front hub/bearing assembly with integrated sensor. Clear ABS codes and road-test after repair.
Sources for this issue (2)
Terrain front and rear hub assemblies integrate the ABS magnetic encoder ring into the bearing's inboard seal, where it sits directly exposed to road salt and moisture. Two distinct failure modes result. The mechanical one is the familiar humming or roaring that rises with speed and changes when loading the bearing in a wide turn. The one that fools owners and shops is encoder-ring failure on a bearing that is still mechanically tight and silent: the wheel speed signal drops out and the vehicle sets a wheel speed sensor code with ABS, traction control and stability control disabled. Replacing only the external sensor does not fix it, because the encoder is part of the bearing. Both generations are affected, with salt-belt vehicles failing earliest.
Common Symptoms
- Humming or roaring that increases with speed
- Noise changes when swerving left or right
- ABS, traction control and stability control warning lights
- Speedometer dropout or erratic reading
- No noise or play but a wheel speed sensor code present
How to Fix
Compare all four live wheel-speed signals and inspect the harness before choosing a hub. If the encoder ring has failed, replace the hub/bearing assembly, not just the sensor. Front/rear, driven/non-driven and 2010-2017 versus 2018-2024 assemblies differ, so match the hub to the wheel position and generation.
Sources for this issue (3)
- C0040 on 2010-2017 GMC Terrain - Right Front Wheel Speed Sensor Fault Causes and Fixesarticle
- 2018-2025 Equinox and 2018-2024 Terrain Wheel Bearing Failure - Diagnosing Noises and Replacement Guidearticle
- LF wheel bearing hub assembly and rear emergency brake shoes - GMC Terrain, Equinox and SRX Forumforum
C0040 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does C0040 mean on a GMC?
C0040 stands for "Right Front Wheel Speed Sensor Circuit." This is a brake/ABS-system code. The right front wheel speed sensor measures how fast that wheel is rotating and feeds that data to the anti-lock brake, traction control, and stability control systems. C0040 means the ABS module found an electrical problem in the right front wheel speed sensor circuit — for example an open circuit, a short, or a signal that is missing or out of range. You'll normally see the ABS and traction/stability lights come on, and those systems may shut off temporarily while your regular brakes keep working. On GMC it is documented across 4 models: Acadia, Sierra 1500, Yukon XL, Terrain.
What are the most common causes of C0040?
The most common causes of C0040 are: Failed right front wheel speed sensor, Damaged, corroded or unplugged sensor wiring/connector, Damaged or rust-clogged tone/reluctor ring, Excessive air gap between sensor and tone ring, Wheel bearing wear affecting an integrated sensor, Water intrusion or corrosion at the connector, Faulty ABS control module (less common). The specific cause varies by vehicle.
How much does it cost to fix C0040 on a GMC?
Published repair estimates on GMC range from $180 to $900 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.