The fog light relay is a remote electrically operated switch and is used in some vehicles wiring to minimise the voltage loss from the battery to the fog lights. In basic fog light wiring systems, the power will travel from the battery to the fog light switch and then to the fog light. When a model> fog light relay is fitted, the voltage foes from the battery to the relay and then directly to the fog light. Because the power has a shorter distance to go, the voltage loss is smaller meaning that the fog lights perform better and put out more light.
The fog light relay is also used to protect the switch from overheating from high amperage, and can offer more flexibility to your circuit. This can mean that you can operate the lights to only come on when other conditions are met such as only lighting when low beams are on.
Relays can fail because of oxidisation, or due to a faulty connection in the relay socket or overheating. If the fog light relay shows signs of failure your fog lights may become dim. This can eventually lead to fog light failure.