How to Fix a Brake Light Fault

Check the bulb or LED assembly, the fuse(s), and the brake light switch first, then inspect the wiring, grounds, and connectors; replace or repair the failed part and verify operation. Brake light issues are usually simple—like a burned-out bulb—but modern vehicles can route the brake signal through a body control module (BCM), so a basic scan and a few quick tests can save time and prevent repeat failures.

What Counts as a Brake Light Fault?

Brake light faults include any condition where your rear brake lamps don’t behave as they should: none of them work, only one side works, the high-mounted center light is out, lights stay on constantly, or they flicker or appear dim. Some vehicles display a “Brake lamp” or “Stop lamp” warning; others only reveal the issue when someone driving behind you notices.

Safety and Preparation

Before working on lighting, park safely, set the parking brake, and if necessary disconnect the battery when accessing tight pedal areas where there may be airbags or sensitive electronics. Gather simple tools so you can diagnose without guesswork.

The following tools and supplies make diagnosis and repair faster and more reliable.

• Owner’s manual (fuse locations and bulb types)
• Replacement bulb(s) or OE-spec LED lamp assembly
• Multimeter or 12V test light
• OBD-II scan tool capable of reading BCM/ABS live data
• Screwdrivers, socket set, trim tool
• Contact cleaner and dielectric grease
• Rags and gloves (avoid touching glass bulbs with bare fingers)

With these on hand, you can confirm power and ground, replace common parts correctly, and protect connections from future corrosion.

Keep these safety considerations in mind while you work.

• Work with the ignition off when probing connectors; use back-probing techniques to avoid damaging terminals.
• Support lamp housings to avoid scratching paint or cracking lenses.
• If your vehicle has a knee airbag near the brake pedal, disconnect the negative battery terminal and wait several minutes before removing the brake switch.

Following these precautions reduces the risk of electrical shorts, damaged trim, or accidental airbag deployment.

Quick Checks You Can Do in Minutes

These fast checks catch the most common causes without any special tools.

1. Have a helper press the brake pedal while you observe all rear lamps, including the high-mounted center light. If alone, use a wall reflection, your phone’s rear camera, or “service mode” on some newer cars to activate lamp tests.
2. Check the relevant fuse(s): labels often include STOP, BRAKE, CHMSL, or BCM. Inspect both cabin and engine-bay fuse boxes.
3. Look for obvious issues: blown bulb filament, cloudy or water-filled lens, loose lamp connector, or corroded socket.
4. Gently wiggle the bulb and harness; if the light flickers, suspect a loose socket or ground.
5. If the lights stay on, lift the brake pedal with your hand; if they go off, the brake switch is misadjusted or sticking.

If these checks point to a bad bulb, fuse, or switch, replacing the part usually restores normal operation. If not, continue with targeted diagnosis.

Diagnose by Symptom

None of the Brake Lights Work

When all brake lamps fail, focus on power supply and the brake switch.

1. Inspect the STOP/CHMSL fuse(s) and replace if blown. If a new fuse blows immediately, there’s a short to ground—often at the lamp socket or trailer wiring.
2. Test the brake light switch at the pedal: verify 12V on the feed wire and switched 12V on the output wire when the pedal is pressed. Many cars use a hall-effect or multi-pin switch; consult wiring colors in the service data.
3. On BCM-controlled systems, use a scan tool to check live data: “Brake Switch A/B” should change from OFF to ON with pedal input. Look for codes like P0571 (Brake Switch A Range/Performance), P0572 (Low), or P0573 (High).
4. Check for a failed ground (common ground point for both tail lamps) and inspect harness sections that flex, such as trunk lid or hatch wiring boots.
5. If the switch and fuses are good but the lamps still don’t illuminate, check BCM outputs and the rear lamp power feed with a multimeter; a failed BCM driver or a relay (rare on modern designs) may be at fault.

When all lamps are out, a fuse, switch, or centralized control issue is most likely; confirm with basic voltage tests before replacing electronics.

One Brake Light Out (Others OK)

A single failed lamp usually indicates a bulb, socket, or local wiring fault.

1. Replace the bulb with the exact type (e.g., 1157, 7443, P21/5W) or an OE-grade LED module. Ensure correct orientation for dual-filament bulbs.
2. Inspect the socket for heat damage or green/white corrosion; clean with contact cleaner and apply a thin film of dielectric grease.
3. Verify ground integrity at that lamp; a poor ground can cause dimming or backfeeding.
4. Check for 12V at the brake pin in the lamp connector when the pedal is pressed. If power is present, the socket or internal lamp circuitry is faulty.
5. On LED tail lamps, the entire assembly may need replacement if the internal driver is failed.

Most single-lamp faults are resolved with a new bulb or socket clean-up; LED assemblies often require full replacement.

Third/High-Mount Brake Light Out

Center high-mounted stop lamps (CHMSL) can be on a separate circuit and fuse.

1. Check for a dedicated CHMSL fuse and power at the lamp connector.
2. Inspect the lens for water intrusion—roof- or spoiler-mounted units often leak and corrode the LED strip.
3. If it’s an LED strip with no serviceable bulbs, replace the entire unit with an OE-equivalent; many require removing interior trim or spoiler fasteners.
4. Verify ground and harness continuity through trunk or hatch wiring boots, a common failure point.

Because CHMSLs are often sealed LED units, replacement of the assembly is common when they fail.

Brake Lights Stuck On

Constantly illuminated brake lights usually trace to the switch or an electrical short.

1. Check for a missing or broken pedal-stop bumper; some cars use a small plastic pad that, if it falls out, keeps the switch “open.”
2. Inspect and adjust or replace the brake light switch; many twist a quarter-turn to release and self-adjust on installation.
3. Scan for P0573 (Brake Switch A High) or related codes; verify live-data states agree with pedal position.
4. Disconnect the switch; if lights turn off, the switch or its adjustment was the cause. If they stay on, suspect a shorted harness or stuck relay/BCM driver.
5. Ensure floor mats or aftermarket pedal covers aren’t interfering with pedal return.

Most “stuck on” cases are fixed by replacing or correctly adjusting the switch or restoring a missing pedal bumper.

Intermittent or Dim Brake Lights

Inconsistent operation points to marginal connections or grounds.

1. Clean and tighten lamp connectors and check for moisture inside the housing.
2. Perform a voltage drop test on the ground and power sides while the brake is applied; more than ~0.2V drop suggests a bad connection.
3. Inspect trailer wiring modules and connectors for corrosion or pinched wires; disconnect the trailer harness to isolate the vehicle side.
4. Verify alternator charging voltage (about 13.8–14.6V with engine running). Low system voltage can dim lamps.

Addressing poor connections and any compromised add-on wiring typically resolves dim or flickering brake lights.

How to Replace Common Parts

Replace a Brake Light Bulb (Dual-Filament Example)

Most conventional brake lamps use an accessible replaceable bulb behind the tail-lamp housing.

1. Open the trunk or hatch and remove the access panel or lamp fasteners (screws/clips).
2. Rotate the bulb socket counterclockwise to remove; pull out the bulb.
3. Install the correct new bulb without touching the glass; align locating pins properly.
4. Before reassembly, press the brake pedal (or use a stick/helper) to confirm operation.
5. Reinstall the socket and housing, ensuring seals sit properly to prevent water intrusion.

Proper bulb type and socket sealing are key to a long-lasting repair.

Replace an LED Tail Lamp Module

Many 2015–2025 vehicles use sealed LED assemblies that integrate drivers and require full-unit replacement.

1. Verify power and ground at the lamp connector to confirm the assembly is the fault.
2. Remove interior trim, then the lamp fasteners; gently free the housing from the body.
3. Disconnect the electrical connector; compare the new unit to ensure correct part number.
4. Install the new lamp, torque fasteners evenly, reconnect, and test.
5. On some European models (e.g., BMW, VW, Mercedes), code the new module or reset lamp adaptations with a scan tool so the BCM recognizes it.

LED assemblies cost more but last longer; ensure correct fitment and perform coding if the vehicle requires it.

Replace/Adjust the Brake Light Switch

Switches vary by vehicle, but most mount above the pedal and are simple to service.

1. Disconnect the battery if working near airbag modules; wait several minutes.
2. Unplug the switch connector. Rotate or unclip the switch from its bracket.
3. If adjustable, set the plunger so the switch closes with slight pedal movement; many self-adjust when installed against the pedal stop.
4. Reinstall, reconnect, and verify lights extinguish with the pedal released and illuminate with light pedal pressure.
5. Clear any related DTCs and confirm cruise control/shift interlock functions as designed.

Correct adjustment prevents both “stuck on” and “no light until hard press” complaints.

Replace a Blown Fuse and Find the Cause

A fuse protects the circuit; if it blows again, there’s an underlying fault.

1. Install the same-amp fuse in the correct slot per the manual.
2. If it blows immediately with pedal press, unplug rear lamps one at a time to isolate the shorted side.
3. Inspect wiring where it flexes (trunk/hatch boot), near trailer connectors, and behind lamp housings for pinched or chafed insulation.
4. Repair damaged wiring with proper splices and heat-shrink; avoid twist-and-tape fixes.

Fuse replacement without addressing the short will lead to repeat failures; isolate systematically.

Using a Scan Tool and Multimeter

Modern vehicles often route the brake signal through the BCM and share it with the engine, transmission, ABS, and ADAS systems. Scanning and measuring prevent misdiagnosis.

• Check live data for Brake Switch A/B status in ECM/BCM/ABS modules; confirm the status changes with pedal movement.
• Read and clear codes: P0571, P0572, P0573 are common for switch faults; some BCMs log lamp-load or short-to-ground codes.
• On vehicles with lamp-out monitoring, verify that coding matches bulb/LED type; mismatches can trigger warnings or disable outputs.

These steps confirm the control side is functioning and help identify software or module-related faults.

Basic electrical tests quickly pinpoint where voltage is lost.

1. With the pedal pressed, measure for 12V at the lamp positive terminal and near 0V drop to ground.
2. If no power, move upstream: lamp connector → rear harness junction → BCM output → brake switch input/output → fuse.
3. Perform continuity and voltage-drop tests on grounds; repair corroded eyelets or body grounds.
4. Back-probe carefully to avoid terminal damage; use proper pin probes.

A methodical “power-and-ground” approach isolates faults without unnecessary parts replacement.

Special Cases and Modern Vehicles

From 2020 onward, many vehicles use smart lamp drivers, PWM control, and diagnostic monitoring that affect how brake lights behave and how they’re serviced. EVs and hybrids may also trigger brake lights during regenerative deceleration.

• LED retrofits: If replacing incandescent bulbs with LEDs, use CANbus-compatible bulbs or appropriate load resistors to avoid bulb-out warnings. Ensure heat from resistors is managed safely.
• Coding: Some brands require coding/adaptation for tail-lamp replacement or for LED vs. halogen configuration.
• EVs/Hybrids: Brake lights may illuminate during regen when decel exceeds a threshold, but the pedal switch still provides the primary stop signal; switch faults can affect cruise control and shift interlock.
• Trailer modules: Aftermarket trailer wiring is a frequent source of shorts; unplug or remove suspect adapters during diagnosis.
• Motorcycles: Vibration and small sockets accelerate failures; check switch at the lever and pedal, and ensure LED conversions are approved and bright enough.

Understanding your vehicle’s specific lighting architecture helps you choose the right parts and procedures and prevents warning messages.

Costs and When to Call a Pro

Brake light repairs are often inexpensive DIY jobs, but integrated LEDs and wiring faults can raise costs.

• Bulb: $2–$15; 10–20 minutes.
• Brake switch: $10–$60 part; 0.3–0.8 labor hours.
• LED tail-lamp assembly: $80–$600+ each, depending on model.
• Diagnostic scan/electrical testing: $100–$200 typical.
• Wiring repair: highly variable; expect 0.5–2.0 hours for common trunk boot fixes.

Seek a professional if fuses keep blowing, multiple systems show brake-switch faults, coding is required, or you’re uncomfortable working near airbags or performing electrical tests.

Prevention Tips

A few simple habits reduce the chance of future brake light faults.

• Test exterior lights monthly or before long trips.
• Keep spare bulbs and fuses in the glovebox if your car uses replaceable bulbs.
• Apply dielectric grease to sockets after cleaning to prevent corrosion.
• Fix moisture leaks promptly; reseal housings and replace cracked lenses.
• Inspect trailer connectors periodically and cap them when not in use.

Routine checks and basic maintenance keep your brake lights reliable and compliant with road laws.

Summary

To fix a brake light fault, confirm the symptom, check fuses, bulbs or LED assemblies, and the brake light switch, then verify power and ground with simple tests. Modern vehicles may require scanning and, in some cases, coding for LED modules. Most issues are resolved quickly with a new bulb, a cleaned socket, a properly adjusted switch, or a repaired wire—follow the steps above to restore safe, legal operation.

How to fix braking system fault light?

What to do when the brake system warning light comes on

1. Check your brake fluid levels. Before you can check your brake fluid levels, you’ll need to locate the master cylinder, sometimes referred to as the brake fluid reservoir.
2. Inspect for leaks.
3. Check the parking brake.
4. Test braking performance.

Can AutoZone fix brake lights?

No, AutoZone does not typically fix brake lights by performing the installation; however, they will sell you the correct brake light bulb, provide guidance on how to install it yourself, and in some cases, offer assistance with simple, accessible bulb replacements by using their basic tools. For complex issues beyond a bulb replacement, such as a faulty brake light switch or wiring problems, you will need to consult a professional mechanic, and AutoZone can help you find one through their Shop Referral Program. 
What AutoZone can help with:

• Part Sales: They stock a variety of brake light bulbs and can help you find the right one for your vehicle. 
• Guidance: AutoZone staff can offer advice and instructions on how to replace the bulb. 
• Simple Installations: Some locations will perform easy, accessible bulb replacements as a courtesy when you purchase parts from them. Call ahead to confirm this service at your local store. 

What you need to do:

• Check Your Owner’s Manual: Your vehicle’s owner’s manual is the best source for information on how to replace the bulb. 
• Bring Your Vehicle: For a free in-store installation, visit your local AutoZone after confirming the service is available, and make sure your car is easily accessible for the task. 
• Use Their Tools: If they perform the installation, you can use their store’s basic tools. 

When to see a mechanic:

• Complex Replacements: If the light assembly is hard to reach or your vehicle requires tools beyond basic screwdrivers, a mechanic is a better option. 
• Persistent Problems: If the new bulb doesn’t fix the issue, or if your brake lights aren’t working at all, there may be a more significant problem, such as a faulty brake light switch or damaged wiring, which requires professional diagnosis. 
• AutoZone’s Referral Program: If the repair is beyond your skill level, AutoZone can help you find a qualified mechanic through their program. 

How much does it cost to fix brake light wiring?

Fixing brake light wiring can cost between $50 and $300 or more, depending on the complexity of the issue, your vehicle’s make and model, and the shop you choose. Simple problems, like replacing a cut wire or a bad bulb, might be on the lower end, while a complete wiring harness replacement could be closer to $300 or higher. Factors like location and whether you use a dealership versus an independent shop will also affect the final price.
 
Factors influencing the cost

• Nature of the damage: A simple cut wire is much cheaper to fix than a complex electrical fault or a damaged wiring harness. 
• Vehicle make and model: Different cars have different parts and wiring systems, which can influence the cost. 
• Labor costs: Hourly rates for auto repair shops vary, with independent shops typically being cheaper than dealerships. 
• Parts cost: The price of a new wiring harness or any specialized components needed will add to the total. 
• Location: Repair costs vary depending on your geographic location. 

Typical repair scenarios and costs

• Simple Wire Repair: Opens in new tabFor a minor cut or damaged wire, a technician can often fix it by stripping the wires, adding a connector, and crimping it. This can range from $70 to $120. 
• Wiring Harness Replacement: Opens in new tabIf the entire wiring harness is broken, you might need to replace it. This can cost between $200 and $300 or more, including the part and labor. 
• Blown Bulb/Fuse: Opens in new tabIf the problem is a blown brake light bulb or fuse, this is a much cheaper and easier fix, costing only a few dollars for the part. 

How to get an accurate quote

• Get a diagnostic: Take your vehicle to an auto repair shop to determine the exact cause of the issue. 
• Ask for a detailed quote: Get a written estimate that breaks down parts and labor costs. 
• Consider independent shops: Independent shops often have lower labor rates than dealerships. 

What does brake light fault mean?

A “brake light fault” can mean several issues, from a simple blown bulb or loose wiring to more serious problems like low brake fluid, a worn brake pad, or an ABS system malfunction. It could also indicate a problem with the parking brake being partially engaged or a faulty brake light switch. You should never ignore a brake light warning; check your parking brake and brake fluid first, but if the issue persists, have the vehicle inspected by a professional immediately, as your braking system is critical for safety.
 
Common Causes

• Parking Brake Engaged: Opens in new tabThe most basic cause is that the parking brake isn’t fully released. 
• Low Brake Fluid: Opens in new tabInsufficient brake fluid can trigger the light, often accompanied by a spongy brake pedal. 
• Worn Brake Pads: Opens in new tabMany vehicles have sensors that detect when brake pads are worn thin and trigger the light. 
• Faulty Sensors: Opens in new tabSensors for brake fluid levels, brake pad wear, or the parking brake can fail, causing a false warning. 
• Wiring or Electrical Issues: Opens in new tabProblems with wiring, loose connections, or a faulty brake light switch can cause a fault. 
• Malfunctioning ABS System: Opens in new tabA fault in the Anti-lock Brake System, such as a wheel speed sensor failure, can also illuminate the brake warning light. 
• A Single Blown Brake Light Bulb: Opens in new tabIf the fault message specifically says “brake lamp fault,” it could just be a failed bulb filament. 

What to Do

1. Check the Parking Brake: Opens in new tabEnsure the parking brake lever or pedal is fully disengaged. 
2. Check Brake Fluid: Opens in new tabLocate the brake fluid reservoir under the hood and check the fluid level. If it’s low, carefully add the correct type of fluid. 
3. Inspect the Brake Light Bulb: Opens in new tabIf your warning is specific to a bulb, check the rear tail lights for a blown filament. 
4. Do Not Drive If You Notice Other Symptoms: Opens in new tabIf you experience any other signs of brake trouble, such as a spongy pedal that goes to the floor or strange noises, stop driving immediately and have your vehicle towed to a service center. 
5. Get a Professional Inspection: Opens in new tabIf the above checks don’t resolve the issue, have the vehicle diagnosed by a qualified mechanic to pinpoint the exact problem.