fault code guide
C0040: Right Front Wheel Speed Sensor Circuit Malfunction
Your car cannot read the speed of the front right wheel due to an electrical circuit fault. Your standard brakes still stop the car, but safety systems like anti-lock braking and stability control are turned off.
How it shows up: The amber ABS and electronic stability control (ESC) warning lights will light up steady on the dashboard, and an OBD scanner will show code C0040. On some vehicles, the speedometer may fluctuate or cruise control will refuse to engage.
Updated 9 November 2026
Can I keep driving?
Service soon
Your car will stop under normal braking, but emergency anti-skid protection is disabled, which increases your stopping distance on wet or icy roads.
What to do right now
- 1Test your brake pedal at low speed in a safe area to confirm the pedal feels firm and standard braking works normally.
- 2Increase your following distance behind other traffic, especially in wet or slippery conditions.
- 3Avoid harsh braking or sudden cornering while the safety aids are offline.
- 4Book an appointment with a local garage to have the sensor and wiring inspected.
Likely causes
Most likely first.
- Likely
Damaged or snapped wheel speed sensor wiring
The thin wiring harness flexes constantly with steering and suspension movement, leaving it vulnerable to snapping or wearing through on road debris.
- Likely
Faulty right front wheel speed sensor
Moisture, road salt, or internal electrical coil failure inside the sensor body prevents it from sending a clean voltage signal to the ABS control module.
- Possible
Corroded or loose electrical connector
Road spray can penetrate the plug seal inside the wheel arch, corroding the tiny metal pins and breaking the circuit.
- Possible
Damaged or rusted reluctor tone ring
The toothed ring or magnetic seal on the wheel hub can crack, corrode, or collect road grime, preventing the sensor from generating a pulse.
- Rare
Failing wheel bearing
Excessive play in a worn wheel bearing can shift the sensor out of alignment with the magnetic ring or damage the tone ring directly.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Perform a brake pedal check
While safely parked with the engine off, press firmly on the brake pedal. It should feel firm; if it sinks to the floor, do not drive, as you have a separate hydraulic issue.
- 2
Inspect the front right wheel arch
Turn the steering wheel fully to the left, switch off the engine, and look behind the front right tyre. Check the thin wire running to the hub for obvious breaks, chafing against the tyre, or loose clips.
- 3
Check the sensor plug connection
Locate the plastic plug clipped to the inner wing or subframe and confirm it is firmly clicked together. If it feels loose or damaged by road gravel, it may simply need reconnecting properly.
- 4
Look for heavy rust or mud buildup
Shine a torch behind the brake disc where the sensor tip mounts near the hub. A heavy coating of dried mud, road salt, or rust flakes can block the sensor signal.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. Whether the fault requires a simple push-fit sensor replacement or a complete wheel hub assembly containing an integrated magnetic ring will have the biggest impact on total repair costs.
Right front wheel speed sensor replacement
- Do it yourself
- €25–85£20–75 in the UK
- At a garage
- €110–230£95–200 in the UK
Sensor wiring harness repair
- Do it yourself
- Garage job
- At a garage
- €90–180£80–160 in the UK
Front wheel bearing or hub assembly replacement
- Do it yourself
- Garage job
- At a garage
- €190–380£160–330 in the UK
When to go to a garage
- Your brake pedal feels spongy, sinks towards the floor, or requires excessive effort to stop the vehicle.
- A red brake system warning lamp illuminates alongside the amber ABS light.
- You hear a loud, continuous humming or rumbling noise from the front right wheel that changes pitch when cornering.
- Inspecting the sensor requires jacking up the car or pressing out a seized sensor bolt that risks snapping in the hub.
Questions drivers ask
Will code C0040 cause my car to fail the NCT or MOT?
Yes, it will fail both tests. An ABS or electronic stability control warning light that stays illuminated on the dashboard is an automatic failure on both the Irish NCT and the UK MOT. The fault must be repaired and the warning light cleared before presentation.
Is it safe to drive home with C0040 showing?
Yes, provided the brake pedal feels normal and only amber warning lights are illuminated. Your primary hydraulic brakes will function normally, but you will not have anti-lock braking or stability control during sudden emergency manoeuvres. Drive gently, leave generous stopping distance, and avoid wet or icy motorways.
Will clearing the fault code with an OBD scanner fix the issue?
No, clearing the code will only turn off the warning light for a few seconds. The ABS computer runs continuous electrical continuity checks, so as soon as the ignition is turned on or the car moves, C0040 will return immediately if the wiring or sensor is faulty.
What is the difference between fault code C0040 and C0035?
Both codes indicate an open circuit, short, or signal failure in a front wheel speed sensor circuit. The only difference is location: C0040 refers specifically to the right front wheel, whereas C0035 refers to the left front wheel.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.