How do I diagnose my heater?

[NC_John]

The temp is controlled only through the blend door (blending fresh cold air with air passing through the heater core). Make sure heater hoses are getting hot, make sure the cable actuated blend door actually moves (follow cable and manually move door) and make sure heater box isn't full of crap blocking the blend door or airflow. Its a really basic system to you ought to be able to find the problem pretty easily.