In this troubleshooting situation, the equipment is a gas pack, and the resident has called for service because the building “isn’t warm at all.” After confirming the conditions reported by the customer and checking to be sure that the filter is clean, you perform an initial check of this rooftop unit, and note that the fault code light on the printed circuit board shows a sequence of three flashes.

When you confirm the code as “High Limit Switch protection device open” you take the next step of checking the wiring diagram, a photo of which is shown in Figure One.