I’d check the coolant level carefully and get it at least on the correct range. Mine has a funny float that sticks up above the mouth of the filling tube. Check the level again after a run to operating temperature. May have to make sure the system has no air in it (bleed it). Thermostat is another possibility. Not a difficult replacement. Dual zone temperature control so may be a good idea to feel the hose into the heater solenoid and both the outlet hoses.