2007 Pontiac Grand Prix heater problem

The dash gauges are notoriously inaccurate. But it is possible that the thermostat is stuck open. Does the upper radiator hose get hot after engine warmed up? Also feel both heater hoses-when engine is warm, both should be hot-indicating the coolant is circulating through the heater core. If heater hoses are hot, and you still have cold air, the temperature blend door on the heater case may not be operating.
Try a new thermostat and see if engine warms up faster and better.

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 .

You have a plugged up heater core. Try pulling the heater hoses of where they go into the firewall, put a garden hose with a spray nozzle in one hose and gently flush the system out. you will need to pull both heater hoses off to do this.

Could be the thermostat in the engine is stuck open , but your engine temp guage would show that the engine is running cooler than usual. Could be a partially blocked heater core , low flow of hot coolant makes for cool air.

clogged heater core very pop prob. check both hoses to firewall from back of engine pass side to make sure they are equal tmep if not it is a bad heater core you can flush these by removing the hoses and blowing them out with air or a garden hose also check to see if you hear a door closing in the vents when switched from hot to cold if you do it is safe to say itis a heater core

is the temp gauge working and showing the engine is warmed up? also check the heater hoses in the engine compartment ensure both are at the same or close to the same temp. if they are not could have plugged heater core. try flushing. possible blend door could be stuck.

With the engine warmed up feel the heater lines going into the firewall. You will probably find that one is very hot and the other only look warm or even cold. If this is the case the heater core needs to be flushed. With the engine cold remove the hoses at a convenient location and back flush the heater core. Reconnect the lines and warm the engine back up with the heat on.

Check your coolant level, if it is ok, it is most likely the thermostat stuck open.

check the temp control switch in the dash