In the fuel injection system, 
Horns, starter motor and fog lamps of the vehicle; relays are used for transmitting electrical signals to the accessories. Relays therefore function by having a low current control circuit open and close a high current power circuit. Sometimes, a part of the car will not work if a corresponding relay is faulty and such relays are usually located in the engine compartment and at other spots in the car. If the relay is thought to be the problem, it can be taken out and either treated by a certain process or by a dealer service or repair shop since a bad relay must be replaced as a complete relay. When trying to test a relay it is better to look at the wiring diagram to make connections correctly; but if there is no access to the wiring diagram, the following will help: Typically, the relays are provided with two terminals for control circuit to connect with the relay coil and the other terminals for power circuit that connects internally on the relay coil when the control circuit is energized. For ease of identification of control and power circuit terminals relays are mostly marked. To check, place one end of a fused jumper wire across the control circuit terminals while the other end to the positive terminal of the battery, and join the other end of another fused jumper wire to ground; the relay should trigger a 'clacking' sound once these connections are made. If it doesn't click, jumper wires may need to be swapped due to polarity a device may have. After installing the jumper wires it depends with the relay markings to ensure continuity between the power circuit terminals. If the relay does not pass any of these tests it should be replaced.