Not likely. It's probably the Water Temperature Sending Unit located on the engine, or the signal processor on the instrument panel (not likely).
Low Coolant. Poor coolant circulation. Intake gasket. Head gasket.
You are OVERHEATING. You most likely have an intake or Head gasket problem.
blown head gasket
parts and labor should run around $650
the Pontiac Montana is known for its manifold gaskets which have rubber rings around the cooling ports. the dexcool system is high pressured and leaks in the rubber rings can cause coolant to escape through the gasket. sometimes this is seen as steam on the outside in little puffs but sometimes the coolant jets inside the engine where it can seize and actually break the crankshaft. have the gaskets replaced with aftermarket parts
It is possible that the gasket could leak between cylinders. The overheating and coolant loss would happen if the gasket was failing around the coolant passages.
Remove the thermostat housing screws and separate the housing, exsposing the thermostat and coolant, remove old thermostat and gasket. Reinstall new thermostat, gasket and coolant.
It is typical for the 3.4 liter motor to develop head gasket leaks. It usually shows up as a low coolant condition and then develops into a temperature problem. You should have it pressure tested to see if it is the radiator or the head gasket. Answer No it is not common for rads to leak. Most likely it is not the rad unless you have actually seen it. It is probably the intake gasket which will leak coolant. This is a very common problem on these vans.
5 to 7 depending on experience.
Exhaust are longer than intake. Exhaust goes thru the loop in the gasket, intakes set outside gasket.
I have a 2000 Pontiac and my was doing the same thing. It turned out to be my heads were warped. You can smell it coming out of your tail pipe if it is your heads.
You will need a new head gasket if you see engine coolant in the oil. I don't know about the thermostat.