A car can overheat while idling if there is not enough coolant in the radiator or there is a malfunctioning fan, radiator or fan belt. Additionally, some cars have a fan clutch that may cause overheating.
To check a car's coolant levels, allow the car to cool and open the radiator cap. Water should be visible at the top of the radiator. If water is not visible, add the appropriate coolant, typically antifreeze and water, to the radiator and replace the cap. Start the vehicle, and let it idle for several minutes to determine if the problem has been resolved.
If the coolant levels are correct, the problem may be a malfunctioning radiator, cooling fan or fan belt. A radiator leak is typically visible from the bottom of the car. Turn the air conditioning off if necessary, fill the radiator and check under the car for water leaks. Replace the radiator if leaks are present.
The fan and fan belt are also part of the cooling system. Refer to the owner's manual to find out the number and placement of the cooling fans. To determine whether the fan is working properly, turn on the air conditioning and check the engine compartment. If the fan is not turning, the issue may be the fan belt or fan clutch. Replace the faulty part to repair the cooling fan.