A relay is basically a electrically controlled switch that uses a low current to activate or deactivate other devices that uses high current. For example, a switch on your dashboard used to turn on high powered devices in your vehicle, like headlamps, foglamps, air conditioning etc
Some of the problems encountered with the relays are : cracked or loose contacts, relays being oversized, or not being set in position etc. The faulty relay usually leads to overheating of contact, wear and tear and ultimate damage
The life of a relay can be determined by the life of its contacts. Some of the factors that contribute to the failure of the relays are End-of life failures, using a relay to switch voltages and currents beyond the specified limit, amount of time that a relay is in operation, accidents, faulty units etc
Whatever may be the reason for failure, it's important that the problem is diagnosed quickly and the failed relays are replaced promptly.