A thermostat can be blamed for both overheating and under-heating. When a thermostat goes bad, it will begin sticking and not opening or closing when certain temperatures are reached. When the thermostat is stuck in the closed position, it restricts the flow of coolant, causing the motor to heat up. If it is stuck in the open position, it will cause continuous flow through the system, never letting the motor get up to operating temperature.
2. Clogged Radiator
The radiator has some of the smallest places in the entire system. When trash and dirt make their way into the radiator, many times it will get stuck on the ends of the tubes, or even lodged in the tubes themselves. This restricts the flow of coolant through the radiator and lowers the amount a coolant that goes through the motor, causing it to overheat. Many people think a flush will unclog a radiator but many times is not the case. Usually, to unclog a radiator it will have to be removed and taken apart to be rodded, which is pushing a solid rob through the tubes to push the clogs out.
3. Low coolant/Leaky System
This is probably the most common reason, but usually easily recognizable. If a system gets low enough on coolant, then there isn't enough to go through the motor and cool it down enough. Usually the case is that there is a leak, small or big, somewhere in the system. The most common components to leak are the radiator and water pump, so it's best those are checked first. Nursing a cooling system and adding water or antifreeze every day to keep it up is never a good idea. By continuously letting it run low, you can put a lot of wear and tear on different parts of the cooling system, especially with all the plastic parts they put in them now. So get it fixed to avoid bigger problems!
4. Bad Head Gasket
Speaking of bigger problems, this one is probably a worst-case scenario. If a vehicle has overheated too many times, it can blow the head gasket. This internal leak will actually push exhaust fumes straight into the cooling jackets on the motor, making them extremely hot. This type of problem is usually caused by another problem that was not fixed in a timely manner, and is one of the most costly on this list.
5. Bad Water Pump
Although rare, a bad water pump can cause a vehicle to overheat. The belt drives the pump which turns inside and directs the coolant forward. In the rare case that the shaft connecting the pulley to the impeller breaks, then the drive belt will no longer be pushing the water, causing very slow and weak flow.