Brake Lights Not Working – Causes, Diagnosis and How to Fix
If your brake lights are not working, the issue is typically caused by a blown fuse, faulty brake light switch, wiring damage or bad bulbs. This guide explains the most common causes and gives you a step‑by‑step diagnostic process to identify the problem accurately.
Symptoms
- Brake lights do not illuminate when pressing the pedal
- Brake warning light on dashboard
- Cruise control not engaging
- ABS or traction control warnings
- Car won’t shift out of park (in some models)
Main Causes
- Blown brake light fuse
- Faulty brake light switch
- Burned-out brake bulbs
- Damaged wiring or connectors
- Faulty rear light assembly
- Corroded bulb sockets
- Aftermarket trailer wiring faults
Step‑by‑Step Diagnosis
1. Check Brake Light Bulbs
Remove the rear light assembly and inspect the bulbs. Replace any burned-out bulbs.
2. Inspect the Brake Light Fuse
Locate the fuse box and check the brake light fuse. Replace if blown.
3. Check the Brake Pedal Switch
Inspect the brake pedal switch and its wiring. Depending on the vehicle, the switch may use multiple signal circuits, so live data or wiring-diagram checks may be more useful than a simple continuity test alone.
4. Check Wiring and Connectors
Inspect the rear harness for damage, corrosion or loose connectors.
5. Inspect Trailer Wiring (If Equipped)
Faulty trailer modules often cause brake light failure.
6. Verify Rear Light Assembly Condition
Cracked housings or water intrusion can cause intermittent brake light failure.
Relevant Fault Codes
- P0571 – Brake Switch A Circuit
- P0572 – Brake Switch A Circuit Low
- P0573 – Brake Switch A Circuit High
When Is a Diagnostic Scanner Useful for Brake Light Problems?
A diagnostic scanner can help when the brake light problem is related to the brake pedal switch, body control module, ABS system or another electronic control circuit.
On supported vehicles, live data can show whether the brake switch signal changes when the pedal is pressed. A basic engine-only code reader may not provide access to the body or ABS modules needed for this diagnosis.
When to Seek Professional Help
- Brake lights fail intermittently
- Multiple electrical warnings appear
- Brake switch tests show no continuity
- Trailer wiring repeatedly blows fuses
- Rear light assembly shows water damage
