Trailer light problems can be confusing and frustrating. Often a defective trailer taillight bulb is replaced, is working fine and then mysteriously quits working the next day. This scenario is not at all uncommon. Trailer light assemblies have a recommended life expectancy of three to five years. Boat trailer owners can expect less, especially for trailers exposed to salt water. Luckily, troubleshooting procedures for a four-wire light system are fairly straightforward.
Unplug the trailer connector from the tow vehicle. Remove the taillight lens covers and the bulbs. Inspect the bulb and bulb sockets for rust and corrosion. The bulb sockets have connectors that are spring loaded. Press them with a screwdriver and make sure the spring has not rusted and is functioning properly. If any problems are found or the lights have excessively weathered, replace them.
Inspect the wire connections from the taillights to the trailer wiring harness. Gently tug on them to ensure a tight connection. Follow the wiring harness to the trailer plug. Look for snags, breaks, rips or tears in the harness. Look for frayed, dry and cracked insulation or exposed wire. If any of these are present, replace the harness.
Inspect the connector at the trailer and at the tow vehicle. Test each circuit at the vehicle and make sure three circuits supply power. Test the parking lights, the left turn signal and the right turn signal. Test the fourth wire for a good vehicle ground. If any are defective, repair them. Plug the connector to operate the trailer lights and verify any needed repairs.
