The ABS (anti-lock braking system) lamp on the dashboard stays on after you ride off, or comes on while riding. ABS uses a sensor at each wheel to read a toothed ring and eases brake pressure if a wheel starts to lock. When it finds a fault it switches itself off and lights the lamp. Your normal brakes still work, but the wheels can now lock. The most common causes are simple: dirt on a sensor or ring, a damaged sensor wire, a blown fuse or a weak battery. The K-Light 125 reference bike has CBS, not ABS, so no ABS values are given.
Is it safe to ride?
Ride with care, as on a bike without ABS: brake early and gently, especially in rain. If the brake lever or pedal also feels soft, sinks or is very hard, do not ride.
Quick checks first
- Read the owner's manual: on many bikes the lamp stays on at start until the bike moves off and passes a low speed. That is normal.
- Switch off, wait, switch on and ride a short straight distance. Does it go out?
- Look at each wheel sensor and toothed ring for mud, metal chips or damage.
- Did someone recently remove a wheel, change a tyre or sprocket, or work on the brakes?
Diagnose it step by step
1Does the lamp go out after riding a short distance at normal speed?
- Yes: normal self-check. No fault.
- No: go to step 2.
2Does the brake lever or pedal feel normal?
- No: go to Brake lever or pedal feels spongy. Do not ride.
- Yes: go to step 3.
3Is a sensor or toothed ring dirty, bent, loose or damaged?
- Yes: clean it, tighten the sensor bracket. A bent or cracked ring or damaged sensor is replaced. Switch off and on, ride, and see if the lamp clears.
- No: go to step 4.
4Is the sensor wire cut, crushed or unplugged (often after wheel removal)?
- Yes: refit or repair the connection. Clean and seal electrical connectors.
- No: go to step 5.
5Is an ABS fuse blown?
- Yes: replace it with the same rating once. If it blows again, there is a short: workshop. Check fuses and relays.
- No: go to step 6.
6Is the battery weak?
- Yes: low voltage can set an ABS fault. Test and charge a 12 V battery.
- No: go to step 7.
7Are the tyres the correct size and pressure, and the sprockets standard?
- No: wrong sizes change the wheel speeds the ABS compares. Refit standard sizes and Check tyre pressure.
- Yes: the fault code must be read with the maker's tool. Go to Diagnose an ABS warning light.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Normal start-up self-check | Goes out after moving off | None |
| Dirty sensor or tone ring | Mud or metal on ring | Clean, refit |
| Damaged or unplugged sensor wire | Cut wire, loose plug | Repair, Clean and seal electrical connectors |
| Blown ABS fuse | Fuse open | Check fuses and relays |
| Low battery voltage | Battery test fails | Test and charge a 12 V battery |
| Wrong tyre or sprocket size | Not as manual | Fit standard sizes |
| Sensor, ring or modulator fault | Stored fault code | Diagnose an ABS warning light |