Relays and contactors are electro-mechanical switches. They operate much like a light switch except they have a coil that becomes energized to turn the switch on or off. Common issues we see with contactors and relays are a burned up coil usually due to a voltage spike such as a lightening strike or due to water leaking on the coil. You can test for voltage between the 2 coil terminals to see if the coil os bad. If you have the correct voltage between the coil terminals and the relay or contactor is not energizing the coil is most likely bad. The other common issue we see is pitted contact points. If the contact points are badley pitted the contactor or relay will need to be replaced. We do not recommend trying to clean or sand the contacts as this usually leads to a bad connection and compounds the problem. If you are not sure which contactor or relay you need please give us a call or send usemail. We will be happy to help you.