My PT Cruiser over heats when idle. This firs happened a few months ago when I was driving and heard the warning bell go on and heard a soft bubbling noise but no smoke comin from the engine. I let it cool for a few hours and check coolant levels. The tank was fine but there was no coolant in the radiator. Now ever since then there has been sufficient coolant in the radiator but the car still over heats when idle after driving for a long time. Some people say thermostat? Can I easily replace this? I will check the fan. If its not working correctly how can i fix it?