- My sonata is overheating . I turned the AC on to make sure the fan relay was working and the fan worked . So I then I hooked the fan up directly so it would run all the time but it still overheated. I've changed the temp coolant sensor and I put a thermostat in . I thought that thermostat may have been bad so I went and got another. I checked this one on the stove with water and it worked just fine.While I had the thermostat out i flushed the system 3 times letting it run for 10 min each time and it didn't overheat then . So I put the new thermostat in with the jiggle pin at the top, but it's still overheating. I have heat coming through the heater core and heat in the car . Water pump doesn't squeal and I have no leaks.