2002 Pontiac Montana Thermostat replacement

Question edited for clarity.
Question moved to model category.

Pontiac Montana Thermostat Replacement Cost Estimate

https://repairpal.com > estimator > thermostat-replaceme...
The average cost for a Pontiac Montana Thermostat Replacement is between $570 and $623. ... Labor costs are estimated between $204 and $257 while parts are priced ...
Or DIY.

you have a GM common problem and that is a bad intake manifold gasket. GM change the gaskets to plastic and they crank and leak causing over heating. here is a picture of one i took from a 2005 pon. bonneville with 74,000 miles on it

You would need to figure out if the thermostat is stuck closed, or not.
Most mechanics would either replace it or take it out and run the engine without it as a test. If the engine still overheats with the thermostat out, it could be a faulty water pump, restriction in the radiator, or other problems.
A blown head gasket should leak coolant out on the ground or into the engine.

whats the cost on getting a thermostat changed,i was gonna get a radatior flush but advised not to until i changed my thermostat it worn out i guess is what im saying,

I believe the Montana has a 3.4 Liter V6 Engine.
This thermostat is located under the exhaust "crossover" pipe that runs between both the front and rear cylinder heads. Divers side.
Remove the heat shields (10mm bolts) and then the nuts on the exhaust studs. (6 total)
Two bolts hold the T stat housing to the intake.

OK FOLLOW YOUR TOP RAD, HOSE TO YOUR ENGINE THAT WILL BE THE THERMOSTAT HOUSING ON A 3.4 IT IS BEHIND EXHAUST CROSSOVER PIPE THE BOOK CALLS FOR YOU TO TAKE THIS OFF. ONCE YOU GET IEXHAUST OFF THERE IS TWO BOLTS ON THE HOUSING PULL THEM PAY ATTEN. TO WHICH WAY THE THERMOSTAT IS SETTING (VERY IMPORTANT) THEN PUT TOGETHER IN REVERSE. P.S I HAVE DID THIS WITH OUT PULLING EXHAUST PIPE.

the cabel from the heat dial to the heater box behind the dash could be snaped or jamed thus not opperating the lever which operates the flap which open and closes the heat box next to the matrix. or if your coolant is brown in colour it could be your matrix is clogged with sedament and needs flushing out. or it could be your water pump not pumping coolant round your engine or it could be a knackerd thermostat i hope these will help you id start with the easyest first thermostat

Where the upper radiator hose connects to the engine you will find the thermostat under that housing. You must drain the radiator and remove the upper hose. Then remove the 2 bolts holding the housing in place. Make sure to pay attention to which way the thermostat is mounted, spring up or spring down.

this would be a defective thermostat (stuck open), change it out and this should cure the issue. disconnect the battery for 5 minutes to clear the engine light.

Have you tried checking the coolant lines going into your heater core to see if there restricted, it could also be a bad water pump, or a worn or loose drive belt. It sounds like you have poor coolant flow goiing through your system. You might want to also check your coolant reservoir cap to see if it is maintaining the proper pressure. Good luck.