Why Your Taillights Look Dim — Common Causes, Diagnosis, and Fixes

If your taillights appear dim, the most common culprits are poor grounding, corroded sockets or connectors, aged or incorrect bulbs, cloudy lenses, voltage drop from wiring or a weak alternator/battery, issues with LED modules, trailer-wiring faults, or aftermarket tint/film. Dim taillights are a safety hazard and often illegal; diagnosing starts with a visual check, verifying correct bulbs, testing voltage, and inspecting grounds and connectors. Below, we break down what to look for and how to restore full brightness.

How Taillights Are Meant to Work

Taillights illuminate at a lower intensity whenever your parking or headlights are on; brake lights illuminate brighter when you press the pedal. Many assemblies share the same housing, but use different filaments, LEDs, or pulse-width modulation (PWM) to achieve different brightness levels. On modern cars, taillights are often sealed LED units controlled by a body control module (BCM). Any fault that reduces voltage or current, blocks light, or degrades the light source or optics will make them look dim.

Most Likely Causes of Dim Taillights

The list below outlines the most frequent reasons taillights lose brightness, ranging from simple cosmetic issues to electrical faults and LED failures.

• Dirty, hazy, or waterlogged lenses: Oxidation, road film, or moisture diffusion cuts light output.
• Aged bulbs (incandescent): Filaments thin and blacken the glass, reducing output before they fail entirely.
• Wrong bulb type or installation: Using the incorrect bulb number, mixing single- and dual-filament bulbs, or mis-seating a bulb can leave only a low-intensity filament lit.
• Corroded sockets/connectors: Green/white corrosion increases resistance, starving bulbs of voltage.
• Bad ground (common ground point failure): A loose or rusty ground strap or body ground causes significant voltage drop and dim lights on one side or across multiple rear lamps.
• Wiring damage or melted pigtails: Chafed wires, pinched harnesses, or heat-damaged connectors limit current flow.
• Weak battery or alternator, or poor charging voltage: System voltage below spec (e.g., <13.3 V while running) will dim incandescent lights and some LED drivers.
• Body control module (BCM) or lighting control faults: PWM or output-stage issues can under-drive the taillight circuit.
• Trailer-wiring harness faults: Backfeeding or corroded trailer connectors can drag down the taillight circuit.
• Aftermarket tint, covers, or wrong-color LEDs: Smoked films, black housings, or red lenses with white LEDs can dramatically reduce visible output.
• LED assembly degradation: Failing driver electronics or partial LED array failure causes one side or segments to appear dim.
• Aged or burned reflectors: Silvering inside the housing can dull over time, especially with heat.
• Contamination: Dust, road salt, or snow buildup over the lens reduces apparent brightness.

Most issues come down to either light being blocked (dirty lens/tint) or electricity not reaching the lamp efficiently (corrosion, ground, wiring, voltage). Identifying which path is affected narrows the fix quickly.

How to Diagnose Dim Taillights

These steps progress from quick visual checks to simple electrical tests you can perform with basic tools. Stop if you’re unsure—electrical systems can be complex on modern vehicles.

1. Compare functions and sides: With the car on, check tail vs. brake vs. turn signals, and left vs. right. Dim on both sides suggests a shared issue (charging, ground, tint); one side points to local wiring or socket/bulb.
2. Inspect lenses and housings: Look for haze, cracks, water droplets, dirt, or snow. Clean with plastic-safe cleaner; note any internal moisture.
3. Verify bulb type and seating (if not sealed LED): Confirm the exact bulb number in the owner’s manual; ensure dual-filament bulbs aren’t rotated incorrectly and that contacts align.
4. Examine sockets and connectors: Unplug and check for corrosion, heat discoloration, looseness, or broken locking tabs. Apply dielectric grease after cleaning.
5. Check grounds: Locate the rear lamp ground points (often a ring terminal to body). Ensure they’re tight and free of rust/paint; clean to bare metal if needed.
6. Test system voltage: Measure battery voltage—engine off (about 12.4–12.7 V) and running (about 13.7–14.7 V). Low running voltage indicates charging issues.
7. Measure voltage at the taillight: With taillights on, measure across the bulb/socket. More than ~0.5–1.0 V below battery/alternator voltage indicates excessive drop. Check voltage drop to ground (<0.2 V is ideal).
8. Wiggle test and harness inspection: Gently move the harness while lights are on; flicker or changes indicate a loose pin or broken wire.
9. Trailer connector check: Disconnect adapters, clean the trailer plug, and retest. Faulty trailer wiring is a common hidden cause.
10. LED assemblies: Look for uneven segments or flicker. If sealed, the fix is usually replacing the lamp unit. Check for relevant TSBs/recalls.
11. Scan for BCM faults (if available): A scan tool can reveal stored codes or disabled outputs affecting lamp intensity.

By starting with appearance, then confirming correct components, and finally testing voltage and grounds, you can isolate whether the problem is optical, mechanical, or electrical.

Fixes That Restore Full Brightness

Once you identify the likely cause, use one or more of the remedies below. Always match the manufacturer’s specifications.

• Clean and restore lenses: Wash, polish oxidized plastic with a headlight/taillight restoration kit, and reseal if needed. Remove snow/ice buildup.
• Replace worn or incorrect bulbs: Install the exact specified bulb number; avoid ultra-blue-tinted bulbs, which appear dimmer through red lenses.
• Repair or replace sockets and pigtails: Clean corrosion; replace heat-damaged connectors; apply dielectric grease to seals and terminals.
• Fix grounds: Remove the ground fastener, sand to bare metal, reattach firmly, and protect with anti-corrosion spray.
• Address wiring issues: Repair chafed or broken wires; ensure proper strain relief and routing away from sharp edges.
• Resolve charging problems: Replace a weak battery or faulty alternator; verify charging voltage is within 13.7–14.7 V.
• Remove illegal tint/obstructions: Peel off smoked films or replace dark aftermarket housings that reduce light output.
• LED-specific remedies: Replace failing LED assemblies; for LED bulb retrofits, use high-quality red LEDs behind red lenses and ensure proper polarity. Avoid relying on load resistors for brightness—they’re for error codes/hyperflash, not output.
• Replace degraded housings: If internal reflectors are dull or peeling, install new or good-quality replacement units.
• Trailer wiring repair: Replace corroded trailer connectors, modules, or splices and add weatherproof covers.

Most fixes are straightforward once you pinpoint the root cause. Focus on restoring clean optics, correct parts, and solid electrical connections with minimal voltage drop.

Safety, Legal, and Practical Considerations

Dim taillights significantly reduce your visibility to drivers behind you and can lead to citations. Many jurisdictions prohibit tinted lenses or covers that reduce luminous intensity. After any repair or modification, step back 20–25 feet at night to confirm the taillights are clearly visible and that brake lights are distinctly brighter than tails. If your car uses a BCM to control lighting, avoid makeshift splices that can cause further faults.

When to See a Professional

Seek a qualified technician if you measure persistent voltage drop you can’t isolate, suspect BCM or PWM control issues, have sealed LED units with intermittent segments, or if corrosion has damaged harness sections that require proper splicing and weatherproofing. A shop can also check for technical service bulletins related to lighting.

Summary

Dim taillights usually come from blocked light (dirty/tinted lenses) or reduced power delivery (bad grounds, corroded sockets, wiring, or low charging voltage). Verify correct bulbs, clean the lenses, inspect and repair grounds/connectors, measure voltage at the lamp, and address any charging or trailer-wiring issues. For LED units, replace failing assemblies and use correct, high-quality components. Restoring full brightness improves safety, legality, and peace of mind on the road.

Are darkened tail lights legal?

In California, smoked taillights must meet certain requirements in order to comply with state laws. The taillights must emit red light, be clearly visible from a distance of at least 500 feet, and comply with the Federal Motor Vehicle Safety Standards (FMVSS).

Why are my lights coming on but very dim?

Dim lights when first turned on can indicate several potential issues, including faulty wiring, overloaded circuits, or problems with the power grid. It could also be due to loose connections, faulty dimmer switches, or even incorrect light bulbs. If the dimming is severe or persists, it’s crucial to investigate further to ensure electrical safety and prevent potential hazards. 
Here’s a more detailed breakdown:
Possible Causes:

• Overloaded Circuits: When multiple high-power appliances are used on the same circuit, the circuit may not be able to provide enough power, causing lights to dim. 
• Faulty Wiring: Worn, corroded, or improperly installed wiring can create resistance, leading to voltage drops and dimming lights. 
• Power Grid Issues: Problems with the local or regional power grid can cause voltage fluctuations, resulting in dimming lights. 
• Loose Connections: Inadequate connections at light fixtures, switches, or junction boxes can interrupt the flow of electricity and cause dimming. 
• Faulty Dimmer Switches: Dimmer switches, especially older ones, can wear out and cause dimming issues. 
• Incorrect Light Bulbs: Using bulbs with the wrong wattage or type for the fixture can lead to dimming. 
• Other Issues: Faulty ballasts (in fluorescent lights), driver issues in LED fixtures, or even overheating can also contribute to dimming. 

This video explains what causes lights to dim and brighten, and how to fix it: 59sFusion ElectricYouTube · May 8, 2023
Troubleshooting Steps:

1. Identify the Problem: Note when the dimming occurs (e.g., with certain appliances, at specific times) and which lights are affected. 
2. Check the Wiring: Inspect accessible wiring for any signs of damage or loose connections. 
3. Test with a Multimeter: If you have experience, use a multimeter to check voltage at the fixture and compare it to the expected voltage. 
4. Replace Bulbs: Try replacing the bulbs with new ones of the correct type and wattage. 
5. Inspect Dimmer Switches: If you have dimmer switches, test them or consider replacing them. 
6. Consult an Electrician: If you’re uncomfortable with electrical work or the problem persists, it’s best to consult a licensed electrician for a thorough inspection and repair. 

You can watch this video to learn how to troubleshoot and fix flickering or dim LED downlights: 59sTC’s Anything GoesYouTube · Jun 25, 2022
Safety Precautions:

• Turn off the power: . Opens in new tabBefore inspecting or working on any electrical components, turn off the power at the circuit breaker. 
• Avoid DIY repairs: . Opens in new tabIf you’re not comfortable with electrical work, it’s best to leave it to a qualified professional. 
• Be aware of potential hazards: . Opens in new tabFaulty wiring and overloaded circuits can pose fire and electrical shock risks. 

How to brighten tail lights?

Using polishing compound and a soft lint free cloth, apply some compound and work it into the surface. You may need to apply the polishing compound a couple times. Once satisfied, clean the light off and ensure there isn’t any left over compound or wax.

Why are my tail lights so dim?

Dim tail lights and fast-blinking turn signals often indicate a faulty or loose ground connection. Inspect the tail light ground wires for corrosion or poor contact. A bad ground can cause voltage drops, making brake lights go dark and turn signals blink rapidly.