Performed heating diagnostic on 2 year old Lennox package unit. System is not heating. Cycled system for heat, circuit board showing open main limit. Filter was extremely dirty. Removed filter and reset, still showing open limit. Tried bypassing limit switch and board is still showing that limit is open. Need to order board and limit switch and return to replace. System not operational upon departure. Will order parts and return to repair. $480 due on completion