Heater is blowing cold air
Likley broken blend door actuator, or air bubble in the cooling system, or clogged heater core, or a bunch of other possible things… considering you told us virtually nothing about your car.
We have had the car a short time. It Drove smoothly until we
tried to use the heater. At first cold air was blowing. Next the heat
came on. Next the car had messages,including check engine,and engine is heating
and will operate at a lower power. We took it to AAA service center. They tested it and
told us the Head gasket is needing to be replaced. Their policy is to only replace the motor
because the car has 123000 miles. A second service center did not find a problem with thhe
head gasket. They added a new battery and added coolent two mechanics test drove it and said
they see no problem with it. We have it at home and the heater isn’t working.
Both of the places you took your car to sound like either crooks or morons… Firstly a blown head gasket does not usually mean replace the engine… unless, perhaps the cost of replacing the head gasket would be equal to or greater than the cost of replacing the engine. The second guys replaced the battery??? Why?? There was seemingly no issues with the starting/charging system from what you’ve said. I’d say use Cartalk’s mechanics data base to hopefully find a honest independent mechanic in your area to have the car looked at. It literally could be a $50 -150 no big deal fix or it could be a $1500 repair. But either way you need a honest trusted mechanic to properly diagnose the issue.
Has all of the coolant leaked out again?
gm 3400 has plastic intake manifold gaskets with rubber seals. anytime you pull intake and see a metal support/intake gasket you know it has been changed. the head gaskets do fail. i changed mine a few months back. but i bought the nox with that issue. not like i paid retail and it failed the next week. my motor had steel intake gaskets but bad head gaskets.
if motor has not run long with a coolant/oil contamination issue than i might go for new headgaskets. but, thats a tough call
but, you might only need intake gaskets
i had coolant bubbling in overflow tank. pretty good clue