Tail Lights Explained: Bulb Sizes, LED Upgrades & Replacing an Assembly
One red housing at the back of your car does four different jobs — tail, brake, turn, and reverse — and each one runs a different bulb. Here's the full tail-light bulb size reference (1157, 3157, 7443, 921, 194), which numbers interchange and which only look alike, why LED turn signals hyperflash and how to stop it, and when to replace the whole assembly instead of a bulb.
Most people think of the thing at the back of the car as “the tail light,” singular. It isn’t. That one red housing is doing four separate jobs on four separate circuits — running lamp, brake lamp, turn signal, and reverse lamp — and on most vehicles each job runs a different bulb with a different number. That’s why “what bulb does my tail light take?” has no single answer, and why so many people buy the wrong one.
This guide is the reference for the back of the car: what each lamp does, the bulb size chart and which numbers actually interchange, why LED conversions hyperflash and how to stop it, and the honest call on when to replace a bulb, a lens, or the whole tail light assembly.
The short version. Read the number moulded into the base of the bulb you pulled out — that’s the truth for your car. Then match it exactly. 3157 and 7443 look identical and do not swap. If you go LED on a turn signal, plan for hyperflash before it happens.
What’s actually inside a tail light
| Function | What it does | Typical bulb | Lens color (US) |
|---|---|---|---|
| Tail / running lamp | Dim red light on whenever the headlights or parking lights are on — marks your car’s width and presence | Dim filament of a dual-filament bulb (1157, 3157) | Red |
| Brake lamp | Bright red on brake-pedal input | Bright filament of the same dual-filament bulb, or a dedicated 1156/3156/7440 | Red |
| Turn signal | Flashing directional indicator | 1156, 3156, 7440, or the bright filament of a 3157 | Red or amber |
| Reverse lamp | White light rearward when in reverse; also a warning to others | 921 (T15 wedge) most commonly | Clear / white |
| License plate lamp | Illuminates the plate — legally required, often forgotten | 194 / 168 (T10 wedge) | Clear / white |
Two structural patterns cover almost every vehicle:
- Combined-function housings run one dual-filament bulb for both tail and brake. The bulb has two filaments at different brightnesses and a base with two contacts — the car chooses which filament to light.
- Separated-function housings give each job its own single-filament bulb. More bulbs, simpler wiring per lamp, and common on vehicles with amber rear turn signals.
Which one you have determines the bulb you buy, and it’s exactly why the manual’s bulb chart lists several numbers per corner. For the same anatomy walkthrough on the front of the car, see Parts of a Car Headlight.
Tail light bulb size reference
These are the numbers that cover the overwhelming majority of rear lighting on cars, trucks, and SUVs sold in North America.
| Bulb | Base | Filaments | Usually used for | Single/dual partner |
|---|---|---|---|---|
| 1157 | BAY15d (bayonet, offset pins) | Dual | Combined tail + brake | 1156 is the single-filament version of the same base |
| 1156 | BA15s (bayonet, single contact) | Single | Turn signal, brake, or reverse on older vehicles | Pairs with 1157 |
| 3157 | Plastic wedge | Dual | Combined tail + brake, or tail + turn | 3156 is the single-filament version of the same base |
| 3156 | Plastic wedge | Single | Turn signal, brake, reverse | Pairs with 3157 |
| 7443 | Plastic wedge (different from 3157) | Dual | Combined tail + brake | 7440 is the single-filament version of the same base |
| 7440 | Plastic wedge | Single | Turn signal, brake, reverse | Pairs with 7443 |
| 921 | T15 wedge | Single | Reverse / backup lamps, cargo lamps | — |
| 194 / 168 | T10 wedge | Single | License plate, side markers, small marker lamps | Same base, different wattage rating |
What interchanges — and what only looks like it does
- 1157 ↔ 1156 and 3157 ↔ 3156 and 7443 ↔ 7440: these are dual/single pairs on the same base. The single-filament member physically fits a dual-filament socket, but it only provides one brightness — put a 3156 where a 3157 belongs and you lose either your running lamp or your brake lamp. Never substitute downward.
- 3157 vs. 7443: not interchangeable. Both are dual-filament wedge bulbs, both look like a glass capsule in a plastic base, and they are the most commonly confused pair in rear lighting. The wedge bases are shaped differently and won’t seat in each other’s sockets.
- 194 vs. 168: same T10 wedge base, different power rating. They’re widely cross-listed and physically interchange; the higher-wattage one runs hotter, which matters in a small enclosed plate housing.
- 3057, 3157, 4157: the same dual-filament wedge family with different wattage/life ratings, commonly cross-referenced. Confirm against your manual before assuming.
The same principle that governs headlight bulbs applies here: the number on the base is the spec. If you want the front-of-car version of this table, see the Headlight Bulb Size Reference Chart and the Cross-Reference & Interchange Guide.
Going LED at the back: what actually changes
LED rear bulbs are a genuinely good upgrade for one reason that has nothing to do with brightness: rise time. An incandescent brake filament takes a couple hundred milliseconds to reach full output; an LED is at full output essentially instantly. At highway speed that difference is a car length or two of extra reaction distance for the driver behind you. Longer life and lower current draw are the bonus, not the headline.
What changes when you swap:
1. Hyperflash on turn signals. The factory flasher relay detects burnt-out bulbs by watching current draw. An LED draws far less than the incandescent it replaced, the relay reads “bulb out,” and it doubles the blink rate. Three fixes, best first:
- LED-compatible (electronic) flasher relay — replaces the thermal relay with one that doesn’t care about load. Cleanest solution where the relay is accessible and replaceable.
- Load resistor wired in parallel with each LED signal — restores the original current draw. Works everywhere, but the resistor gets genuinely hot and must be mounted to metal, away from wiring looms and plastic.
- CANbus-ready LED bulbs with built-in resistance — simplest install, but confirm the specific bulb is rated for your vehicle’s system.
2. Bulb-out warnings on CANbus vehicles. Same root cause as hyperflash, different symptom — the body control module reports a lamp failure on the dash. Same fixes. This is the rear-of-car twin of the front-end problem covered in How to Fix LED Headlight Flicker & Error Codes.
3. Beam spread inside the housing. A tail-light housing is a reflector engineered around an incandescent filament’s position and its all-directions glow. An LED bulb that fires mostly forward can leave the outer edges of the lens dark, so the lamp looks like a bright dot instead of a lit panel. Choose LED rear bulbs with 360-degree emitter coverage, not a single forward-facing chip — the same geometry logic that governs reflector-housing headlight bulbs.
4. Color. Rear tail and brake lamps must show red, reverse white. A red lens with a white LED behind it works, but a red LED behind a red lens is usually brighter, because the lens isn’t filtering away most of the output. Never fit a white or blue bulb where the light escapes rearward unfiltered.
Bulb, lens, or whole assembly?
| Symptom | What’s actually wrong | The fix |
|---|---|---|
| One function dead, others fine | Burnt filament, or corroded socket contact | Replace the bulb; if the new one also fails, clean or replace the socket |
| All functions dead on one side | Fuse, ground, or connector — not the bulbs | Check fuse and the housing’s ground before buying parts |
| Fogging or standing water inside | Failed housing seal or a cracked lens | Reseal if the housing is sound; replace the assembly if the lens is bonded |
| Hazy, faded, chalky red lens | UV oxidation of the plastic, same as headlight haze | Polish and UV-seal, or replace — see Headlight Restoration vs. Replacement |
| Cracked lens, broken mounting tabs, rear-end damage | Physical damage | Replace the assembly |
| Melted/burnt socket, intermittent contact | Overheated bulb or water intrusion corroded the connector | Replace the socket pigtail, or the assembly if it’s integrated |
The pattern that catches people out: water is the real killer. A hairline crack you’d otherwise ignore lets moisture into a sealed housing, and moisture corrodes sockets and grounds until circuits go intermittent. If there’s water inside, treat it as urgent even when everything still lights.
Where to buy which
Tail lights split cleanly across two very different markets, and buying from the wrong one is how people end up with a part that doesn’t fit.
Used OEM assemblies — eBay Motors. A tail light is a vehicle-specific body part: the lens shape, mounting points, connector, and pin count are all model- and often trim-specific. A used OEM assembly pulled from a salvaged car is literally the part your vehicle shipped with. That’s decisive for older vehicles, discontinued models, and trims no longer stocked new, and it’s why the rear of the car is squarely eBay Motors territory rather than a general-catalog purchase. Check the pin count and the year range on the listing before you buy — mid-generation facelifts change tail lights more often than you’d expect.
Bulbs, flasher relays, and load resistors — Amazon. The consumables and the small electrical parts are universal by number, not by vehicle, so they’re a straightforward catalog buy. Browse rear-compatible LED bulbs in Bulbs and electrical fix parts in Wiring & Harnesses.
New full-LED aftermarket assemblies exist for popular platforms and are the route if you want an LED look on a car that came with incandescent lamps. Prioritize DOT/SAE-marked units — an unmarked assembly is the fastest way to fail an inspection.
Legality, briefly
Not legal advice, but the rules are simple and near-universal in the US:
- Rear tail and brake lamps must be red. Reverse lamps must be white. Rear turn signals may be red or amber in the US; much of Europe requires amber.
- Both sides must work, including the license plate lamp — a dead plate light is a genuine, commonly-written ticket.
- Tint reduces output. Federal equipment standards set minimum rear light output; overlay film and spray tint typically push a lamp below it. Buy an assembly certified at the darkness you want rather than darkening a compliant one.
- No white light rearward other than reverse lamps.
Enforcement is state-level and varies, but the equipment standard doesn’t. The same distinction that governs LED headlight legality applies here: a certified assembly is a much stronger position than a modified one.
Related reading
- Parts of a Car Headlight — the front-of-car anatomy companion to this page
- Headlight Bulb Size Reference Chart — the front-lighting size table
- Headlight Bulb Cross-Reference & Interchange Guide — which numbers are the same bulb, and which only look alike
- How to Fix LED Headlight Flicker & Error Codes — CANbus, resistors, and relays in depth
- Headlight Restoration vs. Replacement — lens haze applies to red lenses too
- Are LED Headlights Legal? — the certified-assembly vs. modified-lamp distinction
- Shop by category: Tail Lights · Bulbs · Wiring & Harnesses · LED Headlights
Shopping used or OEM? Try eBay Motors
Live listings for the parts covered in this guide — prices set by the seller.