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Severity: Moderate Code type: Generic (all makes) ✱

C0040 Code: Right Front Wheel Speed Sensor Circuit

The ABS module isn't getting a usable signal from the passenger-side front wheel speed sensor.

Unverified: this manufacturer-specific definition comes from a single source or varies by model. Confirm it with a brand-specific scan tool.

Safety first · Consult a professional

  • Braking system affected: ABS may be disabled. Drive carefully and have it checked promptly.

At a glance

Severity: 3/5 – Moderate3/5

Severity: Moderate

Meaning
Right Front Wheel Speed Sensor Circuit
System
Brakes, ABS & Chassis · Chassis
Safe to drive?
With caution
Typical repair cost
$0–$700
DIY difficulty
Moderate

In the widely used GM numbering, C0040 means the right front (passenger-side) wheel speed sensor circuit: the brake module sees no signal, an erratic one, or an electrical open or short on that corner and turns off ABS, traction control and usually stability control. Many vehicles now have the sensor built into the front hub bearing or read a magnetic encoder in the bearing seal, so a noisy or loose bearing can be the real cause. Be careful with the definition: the newer SAE list names C0040 'Brake Pedal Switch A', and Ford uses C1145 for this wheel, so check the description your scan tool shows. The sister codes are C0035 (left front), C0045 (left rear) and C0050 (right rear).

Can I keep driving? The brakes themselves work normally, but the right front wheel can lock in a hard stop and the car may pull. Avoid emergency-style driving and slippery roads until the sensor circuit is repaired.

How to read C0040

How to read C0040C10203404
  1. C Chassis
  2. 0 Generic (SAE)
  3. 0 Brakes, ABS & Chassis
  4. 40 Specific fault

Symptoms

  • ABS and traction control lights on, often with 'Service StabiliTrak' or an ESC message
  • Speedometer or odometer acting up on vehicles that take road speed from the ABS
  • Cruise control cancels or won't set
  • Light comes on after hitting a pothole or curb on the passenger side
  • Humming that changes when turning left if the hub bearing is failing
  • Transmission shifts oddly on models that share wheel speed data

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Worn front hub bearing with built-in sensor or damaged magnetic encoder (common on hub-integrated designs) 26%
  2. Harness chafed on the strut, brake hose bracket or inner fender, or pulled by suspension travel 23%
  3. Failed right front sensor element 20%
  4. Water and salt in the connector behind the passenger-side wheel liner 14%
  5. Sensor tip fouled with brake dust, metal particles or CV-joint grease from a torn boot 11%
  6. Wrong sensor fitted or ABS module input fault 6%

Diagnosis step by step

Diagnostic flow: C00401Confirm the definition and comparespeeds2Test from the module side3Check an active sensor properly4Inspect the encoder and sensor tip5Check the hub bearing6Repair and verify

  1. Confirm the definition and compare speeds

    Check that your scan tool describes C0040 as the right front wheel speed sensor. Drive in a straight line at about 20-30 mph and compare right front with left front; they should match within about 1-2 mph, while a right front value that drops out on bumps suggests wiring.

    Tools: OBD-II scanner

  2. Test from the module side

    Unplug the ABS module connector (battery disconnected) and measure the right front sensor circuit there, so the reading includes the whole harness. Compare with the left front circuit; a big difference or an open reading while flexing the harness at the wheel points to the wire, not the sensor.

    Tools: Multimeter, Screwdriver

  3. Check an active sensor properly

    Most modern sensors are active (Hall or magnetoresistive) and switch their current, typically between roughly 7 and 14 mA, as the wheel turns. With the key on, confirm the supply from the module, then check for a changing signal while slowly rotating the wheel; an ohmmeter check alone can't judge these sensors.

    Tools: Multimeter

  4. Inspect the encoder and sensor tip

    Remove the sensor and look at the tip and the target through the bore. Clean off metal fuzz and grease, and if grease is present, check the outer CV boot. On bearings with a magnetic seal ring, a scratched or contaminated ring can't be cleaned back to spec.

    Tools: Flashlight, Soft brush

  5. Check the hub bearing

    Raise the wheel, spin it and listen, then check for play at 12 and 6 o'clock. On hub-integrated designs the sensor can't be replaced separately, so a noisy or loose bearing means a complete hub assembly.

    Tools: Flashlight

  6. Repair and verify

    Route the new sensor lead exactly in the original clips, torque the hub and axle nut to spec, clear codes and drive above about 20 mph with a few left and right turns. Recheck live data to confirm all four wheels match.

    Tools: Torque wrench, OBD-II scanner

Where the parts are

Charging system and control modules – C00401234567
Charging system and control modules
  1. 1Battery
  2. 2Alternator
  3. 3Engine computer (ECM)
  4. 4CAN bus
  5. 5Transmission
  6. 6Wheel speed sensor
  7. 7High-voltage battery

Repair options & costs

RepairDIY?Estimated cost
Clean sensor tip and encoder, refit sensor Yes $0–$110
Replace right front wheel speed sensor Yes $90–$300
Splice or replace sensor harness section No $90–$260
Replace front hub bearing with integrated sensor No $280–$700
Replace CV axle or boot if grease contaminated the sensor No $200–$550

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Field data

Chevrolet 5

Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.

Emissions / smog test impact

Depends

C0040 is an ABS code, so it won't fail an OBD emissions test by itself. In states that run a safety inspection, an ABS warning light that stays on can be a reason for rejection.

Frequently asked questions

Which wheel is 'right front'?

Sides are named from the driver's seat, so right front is the passenger-side front wheel on US vehicles. The driver-side front wheel is C0035.

My scanner says C0040 is a brake pedal switch. Which is right?

Both definitions exist. The GM-style list uses C0040 for the right front wheel speed sensor, while the newer SAE list and some Ford and Chrysler-group vehicles use it for brake pedal switch A. Go by the description your scan tool gives for your exact vehicle.

Why does the code come back after I replaced the sensor?

On many vehicles the part that actually fails is the magnetic encoder in the hub bearing, or the wiring along the strut. If live data still shows a dropout after a new sensor, check the bearing and harness before buying more parts.

Can I swap the left and right front sensors to test?

Only if they're the same part number, which is often not the case at the front because the leads differ in length and routing. Comparing live data and resistance at the module is usually the quicker test.

Sources