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My 5 speed 05 civic started to overheat this morning. The gauge went up but I shut the car off before it got completely to the red mark. The problem that I can see is that with the car idleing, the fans (both) will not come on unless you put the heater switch to the on position. Then they both come on. In my opinion the fans should coming on even if the heat switch is off if the car is hot. Is that correct? I might add that I could find no leaks anywhere, no antifreeze coming from the over flow and there was none in the oil. I was also thinking maybe the thermostat. Could you direct me to that. Thanks in advance.
Lots of help here this morning. Anyways I think I may have had a thermastat issue. When the car was over heating the top hose was hot but the bottom was cold. So I just replaced the thermastat and gasket, refilled with antifreeze and took it for a drive. The car is not over heating now but the gauge is going to about 3/4 and staying there. I think there may be some air in there so I will go about trying to bleed that out. Also the bottom hose is getting hot now so the thermastat is working. Any more ideas would help.
Did you buy an OEM thermostat? These cars don't like aftermarket t-stats.
When the car first overheated was the radiator itself (not the overflow tank) low? If the coolant gets too low or there is air in the system it doesn't flow like it should and can give the fan sensor a false reading so it won't come on. Too much air in the system can also prevent the coolant from moving in/out of the overflow tank so fluid would remain in the tank even though the radiator is low.
Hopefully the t-stat fixes the problem but HG failures are common in these cars and don't usually show the typical symptoms.