In the vehicle, relays are used in a number of electrical accessories such as the fuel injection system, 
Horns, starter, and fog lamps where the relay is used to switch the electrical signal to the component. Relays work on the basis of a low current control circuit that can switch on and off a high current power circuit. When a relay is defective, it malfunctions and this affects the function of the corresponding component. Almost all relays are usually fitted in the engine bay fuse / relay box.