I have a 2005 honda civic ex, I have been having problems with the temperature guage and heater. When the car is warmed up the gauge shoots up to high. When I drive on the freeway the gauge reads normal but as soon as I slow down or come to a stop it goes back to hot. Driving on the freeway the gauge will go up and down all day. But on the streets it stays on hot. Now my heater does not work. When the gauge reads hot the heater doesn't work. When the gauge goes to normal, that is when the heater starts working. I already replaced the thermastat and it did not change a thing. Can anyone tell me what the problem is or what should be the next step I should take?

 

Check if your fan by the radiator is ON when temp is hot, also check your coolant level.

 

^^That and check for leaks anywhere, or possibly re-bleed the coolant system to burp any air bubbles out.

Let us know what you find out.