You're driving normally, you cross a pothole or a bump, and suddenly the yellow ABS light shows up on the dashboard. The light stays on even after you turn the car off and back on. The first question every driver asks is whether it's safe to keep driving and how serious the problem is. The short answer is that the brakes still work, but one safety layer has been switched off, and it makes sense to find the cause before an emergency stop on wet pavement catches you out.
What the ABS system does and what you lose when the light is on
ABS, the anti-lock braking system, is the electronics that release and re-apply the brakes on each wheel individually in a fraction of a second. The goal is to keep the wheel from locking up during braking, to keep the car from sliding, and to let the driver steer while braking. Without ABS, if you stamp on the brake in a panic, the wheels lock up and the car slides straight ahead no matter where you turn the wheel.
When the ABS light comes on, the module has switched itself off because it's not getting reliable data from its sensors. The basic brakes still work, purely mechanically, the same as before, the pedal feels the same, the braking force is the same. What you've lost is the electronic anti-lock protection, EBD (Electronic Brakeforce Distribution), and in most cases ESP (Electronic Stability Program) and ASR (traction control on acceleration). That's why the ESP or ASR light often comes on together with the yellow ABS light, since all those systems share the same wheel speed sensors.
The most common cause, wheel sensor and reluctor ring
In nine out of ten cases, the fault is in the wheel speed sensor or in the toothed ring the sensor reads. The sensor is a small plastic part fixed near the wheel hub. A reluctor ring, a toothed ring on the driveshaft or the hub itself, spins next to it. The sensor counts the teeth and that's how it measures the wheel's rotation speed.
Here in Banja Luka this fails for very mundane reasons. In winter the roads are salted with aggressive chemicals, mud and grime build up inside the sensor housing, and stones thrown up by the tyres crack the thin insulation on the cable running to the sensor. It's enough for the sensor to get coated with grease from a torn CV joint boot, or for a layer of rust to stick to the reluctor ring, for the signal to become unreadable. The reluctor ring itself can also crack, especially if it's integrated into the hub or the wheel bearing.
Almost every time a car comes in with the ABS light on, the first thing we do is take the wheel off, pull the sensor out of its housing, and clean both the sensor and the ring. In a good number of cases that solves the problem without buying a single part. Only if the sensor still doesn't produce a signal after cleaning do we move on to replacing it.
Other causes
Apart from the sensor and the ring, the fault can be elsewhere in the system. Here's what else we check:
- Broken or damaged cable from the sensor to the connector on the body. The cable runs along the shock absorber and suspension, and that's where the insulation wears and the wires fatigue from constant flexing. Often the fault only shows up on rough roads, while on smooth tarmac the light goes away.
- Oxidised or wet connector at the sensor plug. Water and salt do their thing, the contacts turn green, the signal drops.
- Blown ABS module fuse. Less common, but it happens. A two-minute check of the fuse box.
- Low brake fluid in the reservoir. The level sensor sends a signal and the system protects itself by shutting ABS down. You top up with DOT 4 fluid and look for where it's leaking.
- The ABS pump or module itself. Rare, but when it happens the repair is more expensive. It usually shows up as a combination of ABS, ESP and a dashboard full of errors.
Worth mentioning that the ABS light sometimes appears after a battery replacement or after a wheel, bearing or driveshaft has been changed, because the sensors were disturbed or the module lost its adaptation.
How we check an ABS fault at the workshop
You can't get far here without an OBD-II (On-Board Diagnostics) scanner. The dashboard only tells you something isn't right, but it doesn't say which wheel. The scanner reads the specific fault code, for example C0035 which points to a problem with the front left wheel sensor, or C0040 for the front right. Only then do you know where to start.
Our usual order is this:
- The fault code is read with the scanner and we note which wheel has the issue.
- That wheel comes off, the sensor comes out, and we visually check for mud, rust or a broken cable.
- The sensor and ring are thoroughly cleaned, the connector is dried and treated with contact grease.
- The code is cleared, the car is test-driven. If the light doesn't come back, the job is done.
- If the code returns, we measure the sensor's resistance with a multimeter and compare it against the factory value. If needed, we replace the sensor, the cable, or in rare cases the wheel bearing with the integrated ring.
The whole check takes anywhere from half an hour to two hours, depending on how stuck the sensor is in its housing. Old sensors are often so rusted into the hub that they have to be broken off when pulled out, and then the housing needs replacing too. The price depends on the actual condition of the car and the workshop, get in touch for an estimate.
Can you drive with the ABS light on
Technically, yes. The basic brakes work, the car stops normally in everyday driving. But there are a few important caveats.
On dry roads at moderate speed you won't feel the difference. The problem comes the moment you have to brake hard, especially on rain, snow, ice or gravel. Without ABS the wheels lock up, the car slides, and the stopping distance gets noticeably longer. The steering doesn't work at that moment either, you can't turn while the wheels are sliding. So the recommendation is to drive moderately, leave a bigger gap to the car in front, and get a diagnostic check done as soon as possible.
There is one scenario where you do NOT drive. If, along with the yellow ABS light, the red brake warning light comes on, the one with an exclamation mark inside a circle, or the light marked BRAKE, that's no longer an ABS problem but a problem with the braking system itself. It could be low brake fluid, brake pads worn down to the metal, or a leak in the system. In that case you stop the car, don't drive any further, and call roadside assistance or a tow truck.
If you're not sure exactly what's lit up on your dashboard, or if the light doesn't go away on its own after a couple of days, drop by the workshop and we'll run a diagnostic on the spot. It's better to check now, while it's a single sensor weighing thirty grams, than to be surprised by your first hard brake on a wet road.