Why One Brake Light Works but Not the Other

If one brake light works while the other doesn’t, the most likely causes are a burned-out bulb or LED module, a bad ground or corroded socket, a side-specific fuse, or a broken wire in the trunk/hinge harness; the brake-light switch is rarely to blame when at least one light illuminates. This issue typically stems from faults that affect only one side of the circuit, and it can often be diagnosed at home with simple checks and a basic test light or multimeter.

How Brake Lights Are Wired

Most modern vehicles feed the left, right, and center high-mount stop lamp (CHMSL) through separate paths. In many cars, the body control module (BCM) switches the left and right brake circuits individually, while the brake-light switch provides a common signal that tells the system you’re braking. On older vehicles, the left and right lamps may be fed by separate fuses with a shared switch. Bulb-based systems typically use dual-filament bulbs for tail/brake functions, while many newer cars use LED assemblies that may require replacing the entire lamp if a section fails.

Most Common Causes

The following list outlines the typical reasons one brake light fails while the other still works, ranked roughly from most to least common. Understanding these possibilities can help you narrow down the issue quickly and efficiently.

• Burned-out bulb or failed LED module: Traditional bulbs often fail after thousands of cycles; many LED assemblies are not serviceable and require replacing the lamp unit.
• Corroded or heat-damaged socket: Moisture or heat can oxidize contacts or melt plastic, breaking the connection for one side.
• Bad ground on that lamp: A loose, rusty, or broken ground wire causes dim or nonfunctional brake lighting on one side.
• Broken wire in the trunk/hinge harness: Repeated opening/closing can fatigue wires inside the rubber boot, especially on hatchbacks and sedans with decklid harnesses.
• Side-specific fuse blown: Some vehicles protect left and right brake circuits separately; a blown fuse can knock out only one side.
• Wrong bulb type or misinstalled bulb: Using a single-contact bulb where a dual-contact bulb is required (or vice versa) can prevent the brake filament from lighting.
• Aftermarket trailer wiring/module fault: Add-on trailer converters can back-feed or interrupt one side, especially if a module fails.
• CAN-bus/load-sensing issues with aftermarket LEDs: Low current draw can confuse monitoring systems, leading the BCM to shut off or pulse a circuit.
• Water ingress in the tail lamp: Leaks can corrode internal tracks or connectors inside the lamp assembly.

While any of these can cause a single-side failure, bulbs, sockets, and harness issues are the most frequent culprits. If the center brake light works, the brake switch and primary signal are almost certainly fine.

How to Diagnose at Home

With basic tools and a methodical approach, you can identify whether the problem lies with the bulb, socket, wiring, or control electronics. The steps below progress from simple checks to targeted testing to help isolate the fault efficiently.

1. Confirm the symptom: Press the brake pedal (use a helper or a heavy object on the pedal) and verify which lights illuminate, including the center high-mount lamp.
2. Check other rear lights on the bad side: If the tail or turn light is also weak or out, suspect a ground or socket issue rather than just the brake filament.
3. Inspect and test fuses: Consult the fuse diagram for left/right brake or stop-lamp fuses; replace any blown fuse with the correct amperage.
4. Swap bulbs side to side (for non-LED): If the problem follows the bulb, the bulb is bad. Ensure you’re using the correct type (e.g., 1157/7443 dual-filament vs. 1156/7440 single-filament).
5. Examine the socket: Look for green/white corrosion, burned contacts, or looseness. Clean light corrosion with contact cleaner; replace a burned or loose socket.
6. Test for power: With the brake applied, use a test light or multimeter to check for 12V at the brake-contact terminal in the socket.
7. Test the ground: With the brake applied, put one probe on the socket’s ground and the other on battery positive; a dim or no light reading indicates a bad ground connection.
8. Inspect the trunk/hinge harness: Gently peel back the rubber boot where wires flex into the trunk lid; look for broken, cracked, or chafed wires and repair with solder/heat-shrink or a pigtail kit.
9. Disconnect aftermarket trailer wiring: Temporarily unplug or bypass trailer modules to see if the brake light returns to normal.
10. For LED assemblies: If accessible, swap the left and right lamp assemblies; if the failure follows the assembly, replace it. If not, suspect wiring or BCM output.
11. Scan for BCM/body codes if available: Some vehicles log circuit faults that can pinpoint an open, short, or overcurrent on one side.

By proceeding in this order—from fuses and bulbs to sockets and wiring—you’ll typically find the issue without guesswork. If diagnostics point to the BCM or complex electronics, professional testing is advisable.

What It Probably Costs

Repair costs vary by vehicle and the specific fault. The estimates below reflect common parts and labor ranges for typical passenger cars and crossovers in the current market.

• Bulb replacement: about $2–$10 for the bulb; $0–$30 labor if done at a quick-service shop.
• Socket replacement: roughly $10–$30 for parts; $50–$120 labor depending on access.
• LED tail lamp assembly: typically $100–$600 per side; luxury models can be higher; 0.5–1.0 hour labor.
• Harness repair at trunk hinge: $5–$20 in materials; $120–$250 labor depending on complexity.
• Trailer module replacement: $30–$120 for aftermarket modules; 0.5–1.5 hours labor.
• BCM diagnosis/programming: $100–$180 diagnostic fee; module replacement varies widely ($200–$800+) and may require programming.

Most owners resolve one-side failures with a bulb or socket fix. Costs rise when LED assemblies or wiring harness repairs are involved.

When the Problem Isn’t the Bulb

Persistent one-side outages after replacing the bulb often trace to grounding or harness fatigue, especially in vehicles where the wiring flexes with every trunk or hatch cycle. Some models have known trouble spots, such as cracked wires in the decklid boot or overheated ground pins in lamp connectors. In rare cases, the BCM’s output driver for one side fails or disables output due to detected faults; scanning for body codes and checking live data can confirm this before parts replacement.

Safety and Legal Considerations

Driving with a nonfunctional brake light reduces your visibility to drivers behind you and may result in a traffic stop or citation. Address the issue promptly, and if diagnostics point beyond basic parts, seek a qualified technician to ensure reliable operation.

Summary

If one brake light works and the other doesn’t, suspect a failed bulb or LED module, corroded socket, bad ground, side-specific fuse, or a broken wire in the trunk/hinge harness. Verify power and ground at the socket, inspect the harness where it flexes, and swap components side to side to isolate the fault. Most fixes are inexpensive unless the lamp assembly or wiring harness needs replacement.

Why is only one of my brake lights not working?

Try changing the bulb, even if it works as a taillight it may be faulty. A brake light bulb has two filaments. If one is blown, the other may still work. Change it, if it still doesn’t work, and the other side does, you have a wiring fault. Get an auto-electrician.

Is it illegal if only one brake light works?

According to Regulation 18, vehicles must have two fully operational brake lights. This legal mandate ensures that other drivers can clearly see when a vehicle is slowing down. Failure to meet this requirement can result in MOT failure or penalties during police inspections.

Why does one brake light work and not the other?

Why does my brake light work on the left side but the right does not? That would depend on the car. Usually, it will be the brake light bulb. Older cars tended to have a twin filament bulb, and often, the brake filament would burn out and leave the tail filament intact.

Can you get pulled over for one brake light not working?

Yes, a non-functioning light can get you pulled over. If an officer watches almost any car long enough, they can find some violation to pull you over. A non-functioning light is an easy spot.