When the thermostat located inside of the coolant system of your 2000 Chevy Monte Carlo stops working, your first clue is the lack of heat produced by the heater. Chevy designed the thermostat in the Monte Carlo to fail in the "safe" position. This prevents the engine from overheating but it also prevents the coolant from reaching the operating temperatures needed to effectively heat the inside of the vehicle. If the thermostat fails in the summer, this is not an issue, but once winter rolls around, you'll quickly decide that it is time to change the thermostat for a new one.
Drive the Monte Carlo onto a set of ramps and secure the parking brake.
Open the engine hood and prop it with the hood prop. Open the radiator cap to release pressure on the cooling system. Do not open the cap on a hot engine. Allow the engine to cool adequately.
Place a drain pan under the petcock located on the bottom of the radiator. Open the petcock with a pair of pliers and drain about 2 gallons of fluid into the drain pan. Close the petcock. This lowers the level of fluid in the system so you do not get drenched in fluid when you remove the upper radiator hose.
Locate the upper radiator hose and trace it back to the engine. It connects to the T-Stat housing with a band clamp. Use the pliers to loosen the band clamp by squeezing the two compression tabs on the clamp together. Pull the hose off the T-Stat housing.
Remove the two bolts that secure the thermostat to the engine block with a 7/16-inch socket and ratchet. Use an extension bar to give you room to turn the ratchet.
Lift the T-Stat housing off the engine block and discard the old thermostat gasket.
Lift the thermostat out of the engine block with your fingers. It sits on a machined lip inside the engine but does not bolt into place.
Wipe the mating surfaces of the engine and the T-Stat housing clean of any old debris with a clean shop rag.
Place the spring end of the new thermostat into the engine block. The end with the pin sticks into the T-Stat housing.
Place a thermostat gasket on top of the engine block and then place the T-Stat housing onto the engine block. Bolt it back into place.
Slide the radiator hose onto the T-Stat housing and secure it with the band clamp.
Drain the radiator fluid back into the radiator via the radiator filler spout. Place the cap back onto the radiator.
