02 grand prix temp rises over 210 then drops back down. heater

Have you checked your cooling fan to see if it is working propely? Let the car idle and get warm and listen for the fan to kick in (if they are electric) or just watch to see if the fan starts to spin faster if it is the old school type with a clutch. Have you replaced the thermostat? A bad one will let the temp rise since it opens too late and then the temp will drop off. But it won't due this only at a light and it will do it infrequently.

Your temp sensor has nothing to do with your heater. The temp sensor needs coolant in order to work properly. So first check the coolant level because if it is low your heater will act the way you describe. Once the coolant gets low enough the heater will blow cold. Another possibility is the radiator cap. After the engine has warmed up it expands when it cools down it contracts. As it is contracting if the cap is weak air can be drawn into the system and cause air pockets to form and will cause the same problem

Could be air in the cooling system , try an bleed the system . On top of the thermostat housing is a bleeder valve , with the engine up to temp. open valve till coolant comes out . Is the coolant full ? What color is the coolant ? Have see where coolant turns like a muddy color an clogges the heater core an engine coolant passages . Check the heater hoses going into the fire wall ,are they hot ? Does your grand prix have manual or automatic climate control ? Duel zone ! Do you hear a clicking sound behind the instrument panel ? Bad temp blend auctuator .

check relay fuse to ur blower under the hood

Sounds like you need a new thermostat, and your A/C needs some freon.

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

What your describing is either a sticking thermostat, a low coolant condition or a blown cylinder head gasket.The code is for coolant temp fault.Not always the temp sensor but it is located next to the thermostat housing.

think it needs more coolant added when engine is cold

I would start by changing your thermostat.

i have the same problem, i was just told to change the temp contol sensor which is only about $13.00 gonna change it friday!!

there is a valve on the water pump and one on the thermostat housing open those up one at a time until you get a little bit of antifreeze come out then tighten them back up. All you will be doing is bleeding the air out of the cooling system. my car was doing the same thing after I changed my thermostat.