Check trailer lights by walking around the trailer with the engine running, testing each light function one at a time, and looking for cracks, corrosion, or burnt-out bulbs

Trailer lights fail silently — you won't know they're out until another driver honks or you get pulled over. A working light system keeps you visible to other vehicles and is legally required in every state. The check takes about ten minutes and needs only a helper, a flashlight, and access to your vehicle's light controls.

You'll test four separate functions: running lights (always on when headlights are on), brake lights (red, brightest when you brake), turn signals (amber, flashing), and backup lights (white, when in reverse). Each one tells you something different about what's working and what needs repair.

Key Takeaways

  • Test trailer lights with the engine running and your helper standing behind the trailer to watch each light function.
  • Check running lights first with headlights on, then brake lights, turn signals left and right, and finally backup lights in reverse.
  • Look for burnt-out bulbs, water inside the lens, corrosion on the socket, and cracks in the housing during your visual inspection.
  • If a light doesn't work, the problem is usually a burnt bulb, a corroded socket, or a loose wire connection at the trailer plug.
  • Test your lights before every long trip and after any time the trailer has been sitting unused for more than a few weeks.

Prepare your workspace and get a helper in position

Park on level ground in a safe location where you can see the back of the trailer clearly. You need another person to stand behind the trailer and watch the lights while you operate the controls from the driver's seat. If you're alone, you can use your phone's camera in video mode — set it up to record the trailer lights, then play it back to see what lit up.

Turn on your headlights and keep the engine running throughout the test. This powers the trailer lights through the tow vehicle's electrical system. Have your helper bring a flashlight; they'll use it to look inside the light housings for burnt bulbs and water damage.

Test running lights with headlights on

Running lights are the small white or amber lights on the front corners of the trailer and red lights on the back. They stay on whenever your headlights are on, whether you're braking or not. Ask your helper to watch both sides of the trailer while you confirm your headlights are on.

Both rear running lights should glow red steadily. If one or both are dark, the bulb is burnt out or the socket is corroded. Your helper should also check the front corner lights if your trailer has them — these are usually amber and should be visible from the side.

Test brake lights by pressing the brake pedal

Brake lights are the brightest red lights on the back of the trailer. They should light up noticeably brighter than the running lights when you press the brake pedal. Have your helper watch both rear corners while you press and release the brake several times.

Both brake lights should respond when ready each time you press. If one is dim or doesn't light at all, the bulb may be failing or the socket connection may be loose. If both lights are out, check the trailer's main power connection at the tow vehicle — the plug may be partially disconnected.

Test turn signals on both sides

Turn signals are amber lights that flash when you set up the left or right turn signal. They're usually located on the sides and back of the trailer. Ask your helper to watch the right side first while you turn on your right turn signal and let it flash three or four times.

The right rear light should flash steadily. Then repeat on the left side. Both sides should flash at the same speed — if one flashes faster than the other, a bulb on the faster side is burnt out. If a signal doesn't flash at all, the bulb is dead or the socket is corroded.

Test backup lights in reverse

Backup lights are white lights on the back of the trailer that come on when you shift into reverse. Put the transmission in reverse (keep your foot on the brake) and ask your helper to watch the rear lights. Both backup lights should glow white brightly.

If backup lights don't work, the problem is often a corroded connection at the trailer plug rather than a burnt bulb, since these lights are used less frequently. Check the plug connection at the tow vehicle first before replacing bulbs.

Inspect the light housings for damage and corrosion

While your helper has the flashlight out, have them look inside each light housing for water, cracks, or a white powdery coating on the socket. Water inside the lens means the seal is failing and moisture is getting in — this will corrode the socket and cause lights to fail soon. A cracked lens should be replaced because water will continue to enter.

Corrosion looks like white, green, or blue crusty buildup on the metal socket or the bulb base. If you see corrosion, the socket needs to be cleaned or replaced. Gently wiggle the bulb while it's in the socket — if it moves easily, the socket is loose and needs tightening or replacement.

Replace bulbs or clean corroded sockets

Burnt-out bulbs are the most common reason for light failure. The bulb will look dark or blackened inside. To replace it, locate the light housing on the trailer, unscrew or unclip the lens cover, and pull the bulb straight out. Buy a replacement bulb that matches the original — the part number is usually printed on the old bulb or in your trailer's manual.

If the socket is corroded, you can clean it with a wire brush or fine steel wool, or you can replace the entire socket assembly. Corrosion will return if water keeps entering, so also check that the lens seal is tight and not cracked. If the problem is a loose wire at the trailer plug, push the connector firmly into the tow vehicle's receiver until you hear or feel it click.

Frequently Asked Questions

How often should I check my trailer lights?

Check them before every trip longer than a few hours and after the trailer has sat unused for more than two weeks. Also test them after heavy rain or if you've noticed water pooling near the light housings. A quick ten-minute check prevents being stranded or cited for a safety violation.

What if one light works but flickers on and off?

A flickering light usually means a loose bulb, a corroded socket, or a corroded connection at the trailer plug. Start by removing and reinserting the bulb firmly. If it still flickers, clean the socket with a wire brush or replace the socket. If all lights flicker together, the problem is the trailer plug connection — disconnect and reconnect it firmly.

Can I drive with one brake light out?

Most states require both brake lights to work, and you can be cited for a safety violation if one is out. More importantly, other drivers may not see you braking if only one light is working, which increases accident risk. Replace the bulb or repair the socket before your next trip.

Why do my lights work sometimes but not always?

Intermittent lights point to a corroded socket, a loose bulb, or a loose connection at the trailer plug. The problem gets worse when the trailer bounces or when temperature changes cause metal to expand and contract. Clean or replace the socket, and check that the trailer plug is fully seated in the tow vehicle's receiver.

What does water inside the light lens mean?

Water inside the lens means the seal around the housing is failing. This will corrode the socket and bulb, causing lights to fail. The lens gasket or the entire light assembly should be replaced. If you ignore it, corrosion will spread and eventually affect multiple lights on that side of the trailer.