96 Buick Park Avenue, heater wont blow hot air. How can I tell if it might be the thermostat or the heater core?

thermostat or low coolant

Look on firewall in the middle behind the engine but on top and you will see the heater hoses. You should see a vacuum type valve. These types seem to go out after a few seasons. Try to move it manually. If you can't then it needs to be replaced.

The heater core on your Park Avenue is under the dash on the passenger side. There are some covers that you will need to remove to access the heater core. I would invest in a service manual for the car these things can be hectic to get out. you can pickup A manual at your local parts store for around 20$ well worth the $

ARE YOU SURE THIS IS THE PROBLEM. Is the heater core leaking if so you would smell antifreeze inside your car the windows would fog on the inside and you would probably have a puddle of antifreeze on the passenger floor board. If its not leaking have the system flushed out
it could be clogged up
My first guess is that the thermostat is sticking open not allowing the coolant to get warm enough to produce much heat id have this checked first cheaper repair and easier to do, also check the antifreeze level, low level could cause this to. hope this helps

You probably created an air bubble, lock in heater core or the hoses that brings hot water to it when the coolant was replaced, you need to flush the system by loosening the heater hose at the highest point while the engine is running and is up to temperature.

Make sure there are no leaks and that the system is full to capacity. After the vehicle has warmed up check radiator hoses for temperature differecial also check heater core hoses. A significant difference in temperature may indicate a plugged heater core or thermostat failure

Check the operation of the blend door motor. The clip may be broken that attaches it .

it is a small compressor for the ride control it pumps air to shocks individually to maintain a stable smooth ride unnecessary but good to have

The cooling thermostat may be failing since this customarily diverts hot water to the heater core first to give a faster response. That isn't an expensive or difficult job to replace it so I would start there. If the system is going to be opened anyhow, I suggest you flush the system first. That may even solve the problem.
Coolant will be lost in the replacement so catch as much as you can to avoid having someone's pet lap up the stuff and die.

this doesnt sound like a thermostat problem, rather a restricted heater core.
The thermostat housing, however, is a cheap cross-your-fingers-fix, and can be located by tracing the upper radiator hose back to the engine. The housing the hose is clamped to is where the thermostat is located. Remove 2 bolts, and replace thermostat.

sounds like the drivers temp door actuator is not working should be located on the drivers side of the air box down by the gas pedal or center dash area