A fan clutch is a device that engages the fan when the engine is hot and disengages it when the engine is cool. This helps to regulate the engine temperature and improve fuel efficiency. A bad fan clutch can cause the engine to overheat or run inefficiently.
There are a few ways to tell if your fan clutch is bad. One way is to listen for a loud noise when the engine is running. Another way is to check the fan for play. If the fan can be moved back and forth, the fan clutch is likely bad.