⚠️ MOT impact: an illuminated ABS warning light is an automatic MOT failure in the UK — get it diagnosed before booking your test.
Common Causes
What Causes This on UK Cars
The exact cause depends on your make, model, year and mileage. These are the most common faults behind an ABS warning light, listed in order of probability on most UK vehicles.
Faulty Wheel Speed Sensor
By far the most common cause. Each wheel has a sensor that reports its rotation speed to the ABS module — if one fails, becomes dirty, or its wiring is damaged, the system can't verify wheel speed and disables itself as a safety precaution. Often triggers the traction control light at the same time, since both systems share the same sensors.
Damaged Reluctor Ring
The sensor reads a toothed ring on the driveshaft or hub — if this ring is corroded, cracked or has missing teeth (common on older or high-mileage cars, especially after a bearing replacement), the sensor sends an inconsistent signal even though it's working correctly itself.
Low Brake Fluid Level
A low brake fluid reservoir can trigger the ABS light alongside the main brake warning light, since some systems use fluid level as part of their self-check. Worth ruling out first — it costs nothing to check.
ABS Pump or Module Fault
Less common but more expensive — the hydraulic pump or control module that actually operates the ABS system can fail on its own, usually at higher mileage. This is one of the causes a wheel-speed-sensor swap won't fix, which is why a proper diagnosis before buying a part matters.