Light issues

I had one do this on a friend's truck. The little drop of solder on the bottom of one of the front signal bulbs broke off, then moved around until it shorted the two contacts together in the socket. One contact is for the tail/running lights, and the other is for the brake / signal lights.

Another common cause of this is when a trailer wiring harness gets chewed up and multiple wires short together. One of the best clues for that is most of the time the dash and running lights only flash on when the left or the right signal is activated, not both.

Probably the most common cause of this is the wrong bulb is in one of the sockets. There will be two contacts on the bottom the signal / tail light bulbs, front and rear. That's a common 1157 bulb. If you work at it, you can stick in an 1156 bulb. That has a single contact and is for a back-up light. The 1156 has two tiny pins directly across from each other near the bottom of the brass base. On the 1157, one of those pins is up higher than the one on the other side.

No. Look inside the socket and you'll see two contacts, side-by-side. The correct bulb is an 1157. Also. The two notches in the socket are staggered, meaning at different distances from the edge. The 1157 has two pins on the side that match those cutouts so that bulb can only be installed one way. That insures the brighter filament for the brake and signals is connected to that circuit, and the smaller filament is connected to the tail light circuit. This bulb sits behind a red lens.

The 1156 bulb has the two pins directly across from each other, it has just one contact right in the middle, and it can be installed two ways. It works the same either way. That bulb is for the back-up lights and has its own socket because it sits behind a clear lens.

By having the different pin location on the 1157, it is supposed to prevent people from being able to install the wrong bulb, but it still happens quite often.