The ABS and brake light come on together when the vehicle detects a fault in the braking system. This includes issues with the ABS module, low brake fluid, faulty wheel speed sensors, or a failing brake booster.
Both lights activating simultaneously signals a shared electrical or hydraulic problem. According to the National Highway Traffic Safety Administration (NHTSA), brake system failures contribute to approximately 22% of vehicle crashes caused by mechanical defects.
NHTSA explains related safety context in NHTSA vehicle safety information.
What Do the ABS and Brake Warning Lights Indicate?
The ABS light indicates a fault in the Anti-lock Braking System. The brake light indicates a problem in the standard braking system. When both illuminate at the same time, the fault is likely systemic rather than isolated.
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These lights are part of the vehicle's onboard diagnostic system (OBD). The system monitors brake pressure, fluid levels, and sensor data in real time.
What Are the 7 Common Reasons the ABS and Brake Light Come On Together?
There are 7 main reasons the ABS and brake light activate simultaneously.
1. Is Low Brake Fluid the Cause?
Low brake fluid is one of the most common reasons both lights activate. The brake fluid reservoir has a sensor that triggers the warning light when fluid drops below the minimum level.
Brake fluid levels drop due to:
- Brake pad wear
- Fluid leaks in the brake lines
- A failing master cylinder
Check the reservoir under the hood. The fluid should sit between the MIN and MAX markers.
2. Is a Faulty ABS Sensor Triggering the Lights?
A faulty wheel speed sensor is a leading cause of ABS light activation. Each wheel has a speed sensor that sends data to the ABS control module. A damaged or dirty sensor disrupts this signal.
Common causes of sensor failure include:
- Road debris
- Corrosion
- Physical damage from potholes
A 2021 study published in the SAE International Journal of Vehicle Dynamics found that wheel speed sensor failure accounts for over 60% of ABS-related fault codes.
3. Is the ABS Control Module Failing?
A failing ABS control module causes both lights to turn on simultaneously. The module processes data from wheel speed sensors and controls brake pressure distribution.
Symptoms of a failing ABS module include:
- Unresponsive brake pedal
- Brakes locking under normal conditions
- Persistent warning lights after restarting the vehicle
4. Is the Brake Booster the Problem?
A failing brake booster reduces braking efficiency and triggers warning lights. The brake booster uses vacuum pressure to amplify the force applied to the brake pedal.
Signs of a failing brake booster include:
- Hard brake pedal
- Increased stopping distance
- Hissing sound when pressing the brake pedal
5. Are Worn Brake Pads Causing the Lights?
Worn brake pads reduce hydraulic pressure in the system, triggering the brake warning light. As pads wear down, brake fluid compensates by filling the caliper space, which lowers the reservoir level.
The NHTSA recommends replacing brake pads when the thickness falls below 2 millimeters.
6. Is There a Blown Fuse in the Braking System?
A blown fuse in the ABS circuit deactivates the system and turns on both warning lights. The ABS system relies on dedicated fuses to protect its electrical components.
Fuses blow due to:
- Electrical surges
- Short circuits
- Faulty wiring
Locate the fuse box (usually under the dashboard or hood) and inspect the ABS fuse with a fuse tester.
7. Is the Parking Brake Engaged?
A partially engaged parking brake activates the brake warning light. Many drivers overlook this as a cause.
Check that the parking brake lever or pedal is fully released. Some vehicles have a faulty parking brake sensor that triggers the light even when the brake is disengaged.
How Do You Diagnose the ABS and Brake Light Problem?
Diagnosing the issue requires 4 steps.
- Check the brake fluid level in the reservoir
- Inspect the parking brake to confirm it is fully released
- Use an OBD-II scanner to read fault codes from the ABS module
- Inspect the wheel speed sensors for damage or corrosion
An OBD-II scanner reads diagnostic trouble codes (DTCs). Common ABS codes include C0035, C0040, and C0045, each corresponding to individual wheel speed sensors.
Can You Drive With the ABS and Brake Light On?
Driving with both lights on is not recommended for extended periods. The standard braking system may still function, but the ABS is likely disabled.
Without ABS, the wheels can lock during hard braking, which increases the risk of skidding. The NHTSA reports that ABS reduces fatal crash risk by 31% on wet roads.
Drive cautiously to the nearest mechanic if both lights are on. Avoid high-speed driving and maintain a greater following distance.
How Do You Fix the ABS and Brake Light Issue?
The fix depends on the cause. There are 6 main solutions.
Cause | Fix Low brake fluid | Refill to the MAX marker; inspect for leaks Faulty wheel speed sensor | Replace the damaged sensor Failing ABS module | Reprogram or replace the module Worn brake pads | Replace pads; inspect rotors Blown fuse | Replace the ABS fuse Failing brake booster | Replace the brake booster
Repair costs vary. A wheel speed sensor replacement costs between $150 and $300. An ABS module replacement costs between $320 and $1,000, depending on the vehicle make and model.
When Should You See a Mechanic for ABS and Brake Light Issues?
See a mechanic immediately if:
- Both lights remain on after checking fluid levels and the parking brake
- The brake pedal feels soft, spongy, or hard
- The vehicle pulls to one side during braking
- Braking distance has increased noticeably
Brake system faults are safety-critical. The Federal Motor Vehicle Safety Standards (FMVSS) mandate that all vehicles sold in the United States include functional ABS as of 2013.
Summary
The ABS and brake light come on together due to 7 main causes: low brake fluid, a faulty wheel speed sensor, a failing ABS module, a worn brake booster, worn brake pads, a blown fuse, or a partially engaged parking brake. Use an OBD-II scanner to identify the fault code. See a mechanic promptly to restore full braking function and road safety.
Jimmy O’Riley is a UK-based mobile mechanic and automotive diagnostic specialist operating out of Bedfordshire, England. He founded O’Rileys Autos in 2011 with a focus on bringing professional vehicle repairs directly to customers at their homes and workplaces.
With over a decade of hands-on experience, Jimmy specializes in ABS diagnostics, brake system repairs, diesel emissions faults, and DPF cleaning. He is recognized across the UK and Ireland as one of the leading specialists in vehicle braking and emissions systems, earning the title “The DPF King” from his growing online audience.
Jimmy documents real-world automotive repairs through his YouTube channel, which has accumulated over 97,000 subscribers and nearly 2,000 published repair videos. His content covers ABS fault diagnosis, wheel speed sensor testing, brake module replacement, and roadside repair procedures across a wide range of vehicle makes and models.
He is active on YouTube, Instagram, and Facebook under O’Rileys Autos.