Check the coolant level, R20A3 engine model, K24Z3 engine model

Check the radiator cap; R20A3 engine model, K24Z3 engine model

Check the operation of the air mix control motor and door

Check the coolant temperature

Check the blower motor unit for obstructions

Check for air leaks around the ducts and vents

Check the inlet hose temperature

If it is COLD, check for restrictions in the hose, a damaged or leaking thermostat, and a damaged or leaking water pump.

If it is HOT, check for restrictions in the heater core. Back flush the heater core using standard commercially available equipment, or replace the heater core.