Need to replace thermostat on pontiac montana van.

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.

put in a radiator flush ,run it for 20 miles it can be a blocked core in both the radiator and the heater.that should clean it. then refill with a 50/50 mix anti freeze then bleed it of air. put your heater on full .run the engine on tick over.then push in the radiator hose to push the air out of the expansion bottle cap .it is off . then refit your cap.

Frost plug, water pump leaking?

find your reset switch and reset it

Thermostat stuck open

Sounds like a head gasket problem to me.

sounds like a leaking head gasket

How do you know the thermostat is ok? If in doubt replace it. Water pumps are a even bigger what if. Did he do a radiator flush? Cooling system flush of the engine? Cooling fans come on at what temp and go off when? Replace the engine temperature sensor? 6 years old is bound to have sensors and such go bad.

your radiator may have frozen the thermostat

How cold is it there and does your engine cool down any while going slow? If it does it may mean you need a hotter thermostat installed, My first thought would be to check the coolant level in the radiator and the coolant reservoir! Hope this helps and Merry Christmas