Symptom
ABS warning light illuminated
The anti-lock braking warning is lit, usually the letters ABS in a circle. It does not mean the brakes have failed — the braking system is hydraulic underneath the electronics and still works normally. What is switched off is the anti-lock function, which only acts during hard braking, so the loss is invisible until an emergency stop, where the wheels can now lock and a locked wheel gives almost no steering. Stability and traction control usually go with it, since they share the same hydraulic unit and sensors. An illuminated lamp is a roadworthiness test failure in most jurisdictions.
Also described as: abs light, abs light on, anti-lock light, abs warning, brake warning light, چراغ ایبیاس روشن شد, چراغ abs, ترمز ضد قفل چراغ, luz abs, testigo abs encendido, luz frenos abs, لمبة ABS, علامة الفرامل ABS, إشارة نظام منع الانغلاق
Some causes mean you should stop driving
5 of the 104 codes that produce this symptom indicate a fault where continuing to drive risks damage or is unsafe. If the symptom is sudden or severe, treat it as one of those until a scan says otherwise.
What this usually means
The brakes have not failed. The braking system is hydraulic underneath the electronics and still works normally — what is switched off is the anti-lock function, which only acts during hard braking. So the loss is invisible until an emergency stop, where a wheel can now lock, and a locked wheel gives almost no steering, which is the part that matters more than the stopping distance.
The commonest cause by a wide margin is a wheel speed sensor or its wiring, and the reason is where it lives: at the hub, in the spray, salt and stone impacts, with a harness that flexes with every suspension movement and every steering input. Stability and traction control usually go out with it — German references make the dependency explicit: the stability system is built on the anti-lock system's data, so losing a wheel speed sensor removes its inputs and it switches itself off. Three lamps, one sensor.
The combination that changes the urgency is the red brake lamp alongside the amber one. German workshop sources flag it directly: together they frequently indicate that electronic brake force distribution has been switched off, so brake pressure is no longer being apportioned between front and rear. The rear wheels can then lock before the front under heavy braking, which spins the car. Amber alone is drive carefully and book it in. Amber with red is not.
A reluctor ring is the failure that defeats a new sensor. The toothed ring the sensor reads is often part of the hub bearing or the driveshaft, and it rusts, cracks or fills with debris — so the sensor is fine, the signal is not, and a replacement sensor changes nothing.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| ABS lamp with stability and traction lamps, car otherwise normal | One wheel speed sensor or its wiring — stability control loses its inputs and switches off too | Read the code, which names the corner. Compare all four wheel speeds live while rolling: the faulty one reads zero or erratically while the others agree. | Renew the sensor or repair the harness. Inspect the reluctor before fitting, because a damaged ring gives the same result. |
| Amber ABS lamp AND the red brake lamp together | Electronic brake force distribution switched off — pressure is no longer apportioned between axles | Confirm the red lamp is not simply the parking brake or low fluid before concluding. Read codes for the brake control module. | Do not drive it normally. Diagnose and repair before use — rear wheels locking before the front under heavy braking is a loss of control, not a longer stop. |
| Sensor replaced and the fault returned | The reluctor ring rather than the sensor — rusted, cracked, or packed with debris | Inspect the ring directly, rotating the hub through a full turn. Watch the live wheel speed for dropouts at particular points of rotation rather than a constant fault. | Renew the reluctor, which on many vehicles means the hub bearing or driveshaft. Clean debris where that is all it is. |
| Lamp appears only above a certain speed, or comes and goes | A marginal signal — a chafed harness flexing with suspension travel, or an air gap opened up by corrosion behind the sensor | Log wheel speeds on a road test and look for a dropout rather than an absence. Inspect the harness where it flexes at the strut and the wheel arch. | Repair the harness and secure it. Clean corrosion from behind the sensor where the gap has been pushed open. |
| Appeared immediately after brake, hub or suspension work | A sensor left unplugged, a harness trapped, or a sensor not seated fully | Retrace the work before reading anything else. The corner named by the code will be the one that was touched. | Reseat and reconnect. Cheapest possible cause and routinely diagnosed last. |
| Lamp on with no sensor codes, or codes for the module itself | The control unit — internal fault, or corroded solder joints in the pump module | Read module-specific codes. Check supply and earths to the unit before condemning it, since low supply produces module faults. | Repair or renew the control unit. Several are repairable rather than replaceable, which is a large cost difference. |
-
ABS lamp with stability and traction lamps, car otherwise normal
- Likely cause
- One wheel speed sensor or its wiring — stability control loses its inputs and switches off too
- How to confirm it
- Read the code, which names the corner. Compare all four wheel speeds live while rolling: the faulty one reads zero or erratically while the others agree.
- What fixes it
- Renew the sensor or repair the harness. Inspect the reluctor before fitting, because a damaged ring gives the same result.
-
Amber ABS lamp AND the red brake lamp together
- Likely cause
- Electronic brake force distribution switched off — pressure is no longer apportioned between axles
- How to confirm it
- Confirm the red lamp is not simply the parking brake or low fluid before concluding. Read codes for the brake control module.
- What fixes it
- Do not drive it normally. Diagnose and repair before use — rear wheels locking before the front under heavy braking is a loss of control, not a longer stop.
-
Sensor replaced and the fault returned
- Likely cause
- The reluctor ring rather than the sensor — rusted, cracked, or packed with debris
- How to confirm it
- Inspect the ring directly, rotating the hub through a full turn. Watch the live wheel speed for dropouts at particular points of rotation rather than a constant fault.
- What fixes it
- Renew the reluctor, which on many vehicles means the hub bearing or driveshaft. Clean debris where that is all it is.
-
Lamp appears only above a certain speed, or comes and goes
- Likely cause
- A marginal signal — a chafed harness flexing with suspension travel, or an air gap opened up by corrosion behind the sensor
- How to confirm it
- Log wheel speeds on a road test and look for a dropout rather than an absence. Inspect the harness where it flexes at the strut and the wheel arch.
- What fixes it
- Repair the harness and secure it. Clean corrosion from behind the sensor where the gap has been pushed open.
-
Appeared immediately after brake, hub or suspension work
- Likely cause
- A sensor left unplugged, a harness trapped, or a sensor not seated fully
- How to confirm it
- Retrace the work before reading anything else. The corner named by the code will be the one that was touched.
- What fixes it
- Reseat and reconnect. Cheapest possible cause and routinely diagnosed last.
-
Lamp on with no sensor codes, or codes for the module itself
- Likely cause
- The control unit — internal fault, or corroded solder joints in the pump module
- How to confirm it
- Read module-specific codes. Check supply and earths to the unit before condemning it, since low supply produces module faults.
- What fixes it
- Repair or renew the control unit. Several are repairable rather than replaceable, which is a large cost difference.
Codes that produce this symptom (104)
Listed with the codes that most typically present this way first — the likeliest place to start.
- C0001 TCS Control Channel A Valve 1 Typical of this code Moderate
- C0002 TCS Control Channel A Valve 2 Typical of this code Moderate
- C0003 TCS Control Channel B Valve 1 Typical of this code Moderate
- C0004 TCS Control Channel B Valve 2 Typical of this code Moderate
- C0010 Left Front Inlet Control Typical of this code Moderate
- C0011 Left Front Outlet Control Typical of this code Moderate
- C0012 Left Front Hydraulic Release Too Long Typical of this code Stop driving High
- C0014 Right Front Inlet Control Typical of this code Moderate
- C0015 Right Front Outlet Control Typical of this code Moderate
- C0016 Right Front Hydraulic Release Too Long Typical of this code Stop driving High
- C0018 Left Rear Inlet Control Typical of this code Moderate
- C0019 Left Rear Outlet Control Typical of this code Moderate
- C001A Left Rear Hydraulic Release Too Long Typical of this code Stop driving High
- C001C Right Rear Inlet Control Typical of this code Moderate
- C001D Right Rear Outlet Control Typical of this code Moderate
- C001E Right Rear Hydraulic Release Too Long Typical of this code Stop driving High
- C0020 ABS Pump Motor Control Typical of this code Moderate
- C0021 Brake Booster Performance Typical of this code High
- C0022 Brake Booster Solenoid Typical of this code Moderate
- C0023 Stop Lamp Control Typical of this code Moderate
- C0030 Left Front Tone Wheel Typical of this code Moderate
- C0031 Left Front Wheel Speed Sensor Typical of this code Moderate
- C0032 Left Front Wheel Speed Sensor Supply Typical of this code Moderate
- C0033 Right Front Tone Wheel Typical of this code Moderate
- C0034 Right Front Wheel Speed Sensor Typical of this code Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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