Some of the electrical accessories in the vehicle which employ relays include the fuel injection system,
Horns, starter, and fog lamps, whereby a low amperage control circuit will switch on a high amperage power circuit. In the case that there is a defective relay, there is no power supplied to the components and defective relays cannot be repaired and has to be replaced. There are three basic types of relays: for the normally-open type; Type A and Type B, while for the five-terminal type. Both Type A and Type B have same internal wiring but different external terminals, while five terminal relays have different internal wiring as well as an extra terminal. To test a relay, the relay should be taken out of the car and then, via an ohmmeter, one should check the continuity. Normally-open type A relays are applied to air conditioning relay, headlight relay and so on, their type can be distinguished by terminals. Others regarded testing as the process of checking whether or not there is continuity between particular terminals when they are active and when they are inactive. As for Type B relay, most of them are normally-open, they include blower motor and rear window defogger, and the identification focuses on terminals. Five terminal relays, used for functions such as moonroof, contain an apparent extra terminal to indicate that they are of this type and testing involves measuring continuity between stated terminals in specific situations.