Here is the deal I have a 98 Catera Love the car but it keeps over heating took in to dealership had the water pump replaced. Ok things are good for a min then it overheates and goes into the red.If i turn the heater in it goes down to normal. Take it in to the dealership again the did a pressure test and it was good all the fans are working no blockage in any hose. Now what??? They said to bring it by when it over heates but they can't seem to find the problem also the theromstat is fine too. Yesterday it did it about 7 times I read somewhere it could be the sensor but don't you think the dealership would of look at that too?
It might be a clogged radiator or a clogged heater core. I would check the radiator to see if its clogged up first since that will be the easiest to get at and fix. The heater core is a royal bitch to get on most cars since you hafta take the dash apart to get at it usually.