You're driving along and an amber light with "ABS" in a circle comes on in the dashboard. It's not a light you see very often, so it's natural to wonder whether the brakes are about to stop working.
The short answer is that your normal brakes should still work. What you've probably lost is the anti-lock system that stops the wheels locking under hard braking. That's still worth sorting quickly, and it will fail an MOT. Here's what the light means, what usually causes it, and when you should stop driving altogether.
What ABS does
ABS stands for anti-lock braking system. Each wheel has a speed sensor that tells the car how fast that wheel is turning. If you brake hard and one wheel starts to lock up, the ABS unit releases and reapplies the brake on that wheel many times a second. That keeps the wheel turning, so you can still steer while stopping.
You'll notice it most in an emergency stop or on a wet or icy road, when the brake pedal pulses and buzzes under your foot. That's normal. It's the system doing its job.
Is it safe to drive with the ABS light on?
If only the amber ABS light is on and the brakes feel normal, you can usually drive carefully to a garage or home. Your car will brake as an older car without ABS would, so the wheels can lock in an emergency stop, especially in the wet. Leave bigger gaps, brake earlier and avoid harsh stops until it's fixed.
Stop and get help if:
- The red brake warning light comes on as well as the ABS light
- The brake pedal feels soft or spongy, or sinks towards the floor
- The car pulls hard to one side when you brake
- You hear grinding or the car doesn't slow down properly
Those point to a problem with the main braking system, not just ABS, and the car shouldn't be driven.
On many cars, the ABS light comes on together with the traction control or stability control light. That's because those systems use the same wheel speed sensors, so if one sensor fails, they all switch off.
Will it fail an MOT?
Yes. Under the MOT inspection manual, an ABS warning light that shows a fault, or one that doesn't come on at all, is a major defect. The light should come on briefly when you switch on the ignition and then go out. If it stays on, or never lights up, the car will fail.
The most common causes
A faulty wheel speed sensor
This is the cause garages see most often. The sensors sit right next to each wheel hub, exposed to water, salt, grit and heat from the brakes. Over time the sensor can fail, or its wiring can crack or corrode. Damage during other work, such as replacing a hub or brake disc, is also common.
A dirty or damaged reluctor ring
The sensor reads a toothed ring, called a reluctor or tone ring, on the hub or driveshaft. If the ring is caked in rust and dirt, cracked or missing teeth, the sensor gets a confused signal and the ABS switches off.
Wiring and connectors
The cable from each sensor runs up through the suspension, where it flexes every time the wheel moves. A chafed wire or a corroded plug can trigger the light, and the fault sometimes comes and goes with the weather or over bumps.
Low brake fluid
On some cars, low brake fluid turns on the ABS light as well as the brake warning light. Check the level in the reservoir under the bonnet. If it's low, the pads may be very worn or there may be a leak, so get it checked rather than just topping it up.
The ABS pump or control module
Less often, the ABS unit itself fails. It's a more expensive repair, so it's worth ruling out the simpler causes first.
Low battery voltage
A weak battery or a fault in the charging system can upset the car's electronics and bring on several warning lights at once, ABS included.
How to find out what's wrong
The quickest way is to read the fault code. The ABS module stores a code that usually says which wheel and which kind of fault it is, for example "front left wheel speed sensor, signal implausible". Many garages can read it in a few minutes, and an inexpensive diagnostic scanner that reads ABS codes will do the same at home.
Once you know which wheel it is, look at the sensor and its cable for damage, check the connector for corrosion, and look at the reluctor ring for rust or broken teeth. If the code clears and comes straight back, the sensor is the usual suspect.
Replacing an ABS sensor
On many cars, a wheel speed sensor is held in with a single bolt and plugs into the loom behind the wheel arch, so it's a manageable job at home. Sensors that have been in place for years can be seized into the hub, though, and may need penetrating oil and patience to remove.
Front and rear sensors are usually different, and so are left and right on many cars. The safest way to get the right one is to match the part number on your old sensor, or the OEM number for your car. If you're not sure, send us your VIN and we'll help you find it.
We stock a large range of ABS wheel speed sensors in our sensors range, along with brake parts for most popular makes.
After fitting a new sensor, the light may go out by itself after a short drive, or the fault code may need clearing with a scanner.
If your brakes are also making noises or feeling different, our guide to worn brake discs and pads covers the signs to look for.
This article is a general guide only. Brakes are a safety-critical system. If you're unsure about the cause of a warning light, have the car checked by a qualified mechanic.