There are few things that can happen to cause problems with trailer lights, and all of them will likely happen at least once during the life of the trailer. At times, the problem is obvious, such as a broken wire. Other times it will be necessary to spend time with a test light to determine the cause of a problem.
Verify the problem and determine which lights are not working properly.
Check for the proper operation of the tow vehicle lights.
Using a test light verify that each pin of the tow vehicle trailer connector is powered as you test the circuits individually. Make sure the vehicle key is on when testing for turn signal power. Test in this order: parking lights on, pin will have steady power; left turn signal, one pin will have pulsing power; right turn signal, a third pin will have pulsing power. The exposed pin will have no power as it is a ground connection. If no power is found when a circuit is on check for a blown fuse in the tow vehicle. Some vehicles have fuses just for the trailer circuits.
Check the trailer ground connection if no lights are working and power was verified at the trailer connector, or if the running lights go out when the tow vehicle brake pedal is pressed. Don't just visually check the connection, actually remove the screw holding the terminal to the trailer. Remove any rust at the connection. Replace the screw with a new one of the same size and type when reattaching the ground terminal to the trailer frame.
Check the lights again. If the problem persists in the same manner check the tow vehicle's trailer connector ground connection. The ground wire on both the trailer and vehicle will be the white wire.
Check the connections at the troublesome light. This is usually a butt connector joining the pigtail from the light to the wire on the trailer. If any corrosion is seen in the connector cut it out and replace it with a new one.
Remove the lens and check the bulb. (This may not be possible on some trailers as the lamp is a sealed unit.) Use a new bulb if the light is still out, remove the bulb and check for power at the center terminal in the bulb socket. There will be two center terminals in most lamps, one for the tail light filament and one for the turn/stop filament,check them both for power. If power is found here with your test light, and the bulb is a known good one, the problem is the ground connection for that lamp. Remove and clean the ground terminal at the trailer frame. Some lamps make the ground connection through the studs which hold the lamp in its bracket. A bad ground with this arrangement usually requires replacement of the entire lamp.
Check the wire from the trailer connector to the effected light if no power was found in the bulb socket and there was power at the tow vehicle trailer connector. Replace the wire if bare spots are found or there is no continuity in it.
