It's probably a head gasket problem.
has the red coolant in a 2005 monte carlo been changed since reports on it deteriorating the head gasket?
how to remove the engine of the 2001 monte carlo step by step
The low coolant sensor is bad or disconnected.
On a 2001 Monte Carlo it is located on the radiator just below the radiator cap.
Check your coolant level...
See a dealer
Under the thermostat
Depends on what engine and what year Monte Carlo you're referring to...
There was no Monte Carlo in 2008, their final year was 2007.
A coolant flush on a 96 Z34 Chevy Monte Carlo helps greatly with increasing the efficiency of the cooling system. It removes buildup and contaminants from the system.