1. Low Coolant By a large margin, the most common cause for engine overheating is simply a low coolant level. Your engine's cooling system relies on coolant to circulate and remove heat from the engine. If you don't have enough coolant in there to do the job, heat will build up and your engine will overheat. No amount of running the heater in the summer will help if you don't have enough coolant in the radiator to transfer that heat. By far, the first thing you should do if your engine seems to be running hot is check your coolant level.
2. Electric Cooling Fan Failure If you have an electric cooling fan that isn't coming on, this can cause your engine to overheat. The electric cooling fan draws cooler air through your radiator when your car isn't going fast enough to ram it through from the front. You can test this by letting your car idle long enough for the engine to heat up. If you've been having an overheating problem in traffic, keep an eye on your temperature gauge. When it starts creeping into the danger zone, look under the hood to see if your electric fan is running. If it's not, you'll need to figure out why. There are two usual causes to this:
2a. Bad Electric Fan: Sometimes your fan motor will just burn out and your fan won't come on at all. To test this, find your radiator fan switch and disconnect the wiring harness. Get a jumper wire and insert it into both contacts, your fan should come on. Another way to test the fan is to turn on the air conditioning. Most, but not all, cars activate the cooling fan at either a medium or high speed when you turn on the AC.
2b. Bad Radiator Fan Switch: There is a switch that tells your cooling fan to come on when your coolant reaches a certain temperature. The easiest way to test this switch is to disconnect the wiring harness and then run a jumper wire across the harness contacts. If the fan comes on, you need to replace the switch.
3. Thermostat Not Opening The most common symptom of a failed thermostat is overheating at highway speeds. Your engine may be able to stay cool at low speeds because it's not working that hard, and therefore not creating as much heat. But at highway speeds your engine needs lots of coolant flowing through. If the thermostat doesn't open, there isn't enough flow to keep things cool, and you'll find yourself looking more like a steam ship than a sedan going down the highway.
4. Broken Fan Belt There are still lots of engines out there which have a fan belt to drive the engine cooling fan. If you see a belt attached to your fan, you're in this club. The good news is your repair is always cheaper than the electric guys. You can replace your fan belt easily if it's broken.
5. Clogged Radiator If your car has more than 50,000 miles on it, your radiator could start getting gummed up. You can avoid this and other problems associated with old coolant by flushing your radiator every year. There's nothing good about an overheating problem. If your engine is running hot you should try to fix the problem as quickly as possible. A hot engine can do damage to itself, even if you aren't going into a full overheating. Check your oil regularly to be sure you are providing adequate lubrication to your engine, everything you can do to reduce heat buildup helps! If you've checked these more common causes of overheating and don't see an answer, check out this advanced overheating troubleshooting guide for more answers!