Much like a computer contains fans inside to keep it from overheating, many cars have electric fans mounted to the radiator to help cool the engine compartment and increase air flow. These can be an important component, especially in high-performance vehicles, so drivers should take time to understand them.
The electric fan is almost always mounted to the radiator itself. There are two types of fans: pushers and pullers. A puller fan is the more efficient of the two, but it's also larger, so many engines do not have enough space for it. These are mounted on the back of the radiator, near the engine, and pull air through the grille and over the radiator. A pusher engine is the exact opposite - mounted on the grille and pushing air over the radiator.
Fans are connected to the car's electric system, and the stock versions that come with a vehicle are likely hooked up to a temperature sensor as well. This means they only come on if the engine reaches a certain temperature. This can somewhat complicate things if there's an issue with the fan, as a driver won't immediately know if there's a problem with the fan or the thermostat.
There are also many aftermarket electric fans to choose from for those who do not have a vehicle that came with a fan or who want increased performance. The high-end versions of these come with their own temperature sensors, some of which may even be programmable so the driver can set custom temperature ranges for fan usage and strength. Some fans will instead provide a switch that can be placed in the passenger compartment, which will let the driver manually control the fan themselves.
To diagnose an issue with an electric fan, you'll have to test to figure out whether the issue is emanating from the fan itself or the thermostat controls, or possibly the wiring. If you have an aftermarket fan, check the wiring first - they should have a positive and negative wire. If these are hooked up backwards, the fan will actually rotate backwards and be ineffective.
Your fan should come on when you turn on your car's air conditioning, so it's pretty easy to check and see if it's working properly. However, if it doesn't come on, you need to diagnose the exact issue. To do this, hook the positive and negative wires up directly to the battery. If your fan comes on and works normally, you know it's not the fan's motor, and the issue likely lies in the thermostat control or perhaps the wiring that the fan is hooked up to.