C0040
ChassisRight Front Wheel Speed Sensor Circuit Malfunction
The ABS module has lost a reliable signal from the wheel speed sensor on the right front corner. That sensor tells the system how fast that wheel is turning, and without it the ABS and ESP can't do their job, so they shut down to be safe. Your brakes still work like a normal non-ABS car, but the clever stuff that stops you skidding under heavy braking is off until you sort it. On most UK cars this turns out to be the sensor itself or its wiring getting a battering from years of road salt and spray right behind the wheel.
ⓘ Information only. This page provides general educational information about fault code C0040. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does C0040 mean?
C0040 is a Chassis (ABS, traction control, steering) fault code. It indicates: Right Front Wheel Speed Sensor Circuit Malfunction.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • ABS light on the dash, usually the first thing you notice
- • Traction control and ESP/stability lights often come on alongside it
- • ABS doesn't engage when you stomp the brakes hard, the pedal just goes solid with no pulsing
- • On some cars the speedometer flickers or drops out because the same sensor feeds it
- • A few vehicles throw a warning chime or show a 'service stability system' message
- • Stability control no longer cuts in on a slippery roundabout or wet bend
Possible causes
Causes commonly associated with C0040, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed wheel speed sensor, by far the most common on this corner. They sit right in the firing line for water, salt and stones, and the internals just give up
- 2. Corroded or chafed wiring near the sensor, where the loom flexes with the suspension every time you go over a bump. Common on older Fords and Vauxhalls
- 3. A clogged or damaged tone ring (reluctor ring). On many cars it's the magnetic ring built into the wheel bearing, and brake dust or rust on it confuses the sensor
- 4. Wheel bearing play opening up the air gap between sensor and ring, so the signal goes intermittent. Often comes with a droning noise from that corner
- 5. Dirty or corroded plug at the sensor connector, water gets in and the resistance goes haywire
- 6. A short to earth or to power in the circuit, less common but it does happen after botched bodywork or a bumper repair
- 7. Faulty ABS module, rare and the last thing to suspect once everything else checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Plug in a scan tool and read the live wheel speed data from all four corners, then drive it. The right front reading dropping to zero or going erratic while the others stay steady tells you straight away the fault is real and where it is
- 2. Get the front right wheel up and have a proper look at the sensor, its wiring and the plug. Pull the connector apart and check for green corrosion or water sitting in it. Wiggle the loom where it runs near the strut, that's where chafing hides
- 3. Spin the wheel by hand and check the tone ring through the sensor mounting. Look for rust scale, missing teeth or brake dust packed onto it
- 4. Check the resistance of a passive sensor with a multimeter against the manual spec, typically somewhere between 800 and 2,500 ohms. An open or short circuit reading means the sensor or its wiring is done
- 5. Rock the wheel top to bottom to feel for bearing play. A worn bearing widens the air gap and gives an intermittent signal, and on integrated units the tone ring is part of the bearing anyway
Common questions about C0040
How do I know if it's the sensor or something else like the wiring or bearing? +
Live data on a scan tool is your friend here. Drive the car and watch the right front speed reading. If it cuts out the moment you hit a bump or a particular bit of road, suspect the wiring or connector first. If it's dead all the time from the off, the sensor or its plug is the likely culprit. If you also hear a droning or humming from that wheel that gets louder with speed and the signal is patchy, get the bearing checked, because a worn one moves the sensor away from the ring. A quick resistance test with a multimeter sorts a dead sensor from a wiring fault in a couple of minutes.
Can I replace or clean it myself? +
Plenty of people do. If the sensor unbolts with a single fastener you can fit a new one for £20 to £70 a part in twenty minutes, just take care not to snap the old one off in the hole. Cleaning is worth a go first if the fault is intermittent. Pull the sensor, wipe the tip clean of metal fuzz, and scrub any rust off the tone ring with a brass brush. Where it gets fiddly is the sensor that's seized into a corroded hub or one that's part of the wheel bearing assembly, and that's a bigger job better left to a garage with the right pullers.
If I just clear the code, will it stay gone? +
Only if the fault was a one-off, like a stone briefly jammed near the ring or a loose connector that's since settled. Clear it on a sensor that's actually failing or a wire that's broken and the light will be back within a mile or two of driving, because the module checks all four wheels constantly. Clearing the code is fine for confirming you've fixed the real problem, it's no use as a fix on its own.
What's the risk if I just leave the light on and keep driving? +
Day to day your normal brakes work, so you won't notice much in the dry. The danger shows up the moment you have to brake hard on a wet or icy road, or in an emergency stop. With ABS off, that wheel can lock and you lose steering control just when you need it most, and the ESP won't catch a slide on a greasy roundabout. It'll also fail the MOT with the light on. Get it sorted before winter rather than finding out the hard way on a slippery motorway slip road.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →