Have heat sometimes

I have a 2004 Pontiac Aztek, sometimes I have heat and sometimes I don’t. About a month ago I took it in for a coolant flush, had heat ever since till about a week ago and the heat went completely out. Yesterday we changed the thermostat and now when I turn the car on the temperature gauge goes all the way to the top, so I shut off the heat and about 15 min later I will turn it back on hand have heat. It will blow freezing cold air so i turn it off the temp gauge goes back down and when i turn it back on 15 min later I have full heat. What could be the problem?

After the coolant flush did you check the fluid level in the overflow tank? Did you add any fluid? It seems you have an air bubble in the system. Air is usually expelled into the overflow tank and then coolant drawn back into the system as the car cools off, meaning daily checks of the overflow tank level and topping off as needed until it is stable.

You might also have a leak somewhere and lost coolant is replaced by air. You need to be sure there are no leaks, and then top of the coolant level as per above until you have no air in the system. During the top off process you need to make sure the heat is on and the temp knob set to a warm temp and that will purge air out of the heater core. Air trapped in the heater core will stop the coolant flow and therefore no heat.

+1 to UncleTurbo’s comments. Air in the system and/or coolant loss.

+2 to Turbo… Step one would be purge all the air out of the system…On some vehicles, this can be tricky because the top tank of the radiator is lower than the heater core…

+1 to Caddyman. I have had good luck purging coolant systems of air by parking nose up on an inclined driveway. Just run the engine with the radiator cap off until it reaches operating temperature. Air in the heater core can be difficult to purge on some vehicles.

Uncle Turbo nailed it…YOu have an air pocket in your system…and unless you have a bleed nipple somewhere the only way to bleed it out may be several hot and cold cycles of the engine…with a FULL Overflow tank so it can pull in fluid when needed and expell what it doesnt…

Usually you have a bleed nipple on the system…No? Check and bleed that thang… This is a great way to blow a head gasket