2008 Chevy Impala

Hello,

Beside thermostat, it could be:
*Low coolant: external leak (inspect carefully all the hoses, thermostat housing, water pump, intake gasket, radiator, etc), internal leak (coolant in oil or vice versa, transmission fluid in coolant, a lot of white smoke coming out of tailpipe, all signs of internal leak).
*bad water pump
*Clogged system (flush)
*bad hose
*bad tensioner/loose serpentine belt
*cooling fans don't come on
*bad radiator cap
*wrong timing

Pressure test your cooling system; that'll tell you if you're dealing with a leak.