Heat problems - 05 Buick Lacrosse CXL

•2005 Buick Lacrosse CXL 3.8L V6, w/around 210,000 miles

•Check engine light is on - Code P0128: Engine coolant temperature below thermostat regulating temperature

•Heater blowing cold air

•Temperature gauge reading just above quarter hash mark when normally reads at the half hash mark

•Inlet hose is warm, outlet hose is cold

•Coolant has been checked and is not low

•Started smoking this morning and vehicle smelled like antifreeze

Any ideas on what the problem is and how to fix it?