Without Climate Control

Insufficient heating

Check the coolant level Testing and Inspection

Check the radiator cap Testing and Inspection

Check the coolant temperature during normal operation

Check the heater core inlet hose temperature:

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

If it is HOT, check for restrictions in the heater core. Back flush or replace the heater core

Do the air mix control motor test Air Mix Control Motor Test

Check the blower motor unit for obstructions

Check for air leaks around the ducts and vents

Also check for

HVAC DTCs How To Troubleshoot The Heating And A/C Systems (Without Climate Control)

Also check for

Damaged cylinder head gasket