Someone told me they thought it was the High Limit Switch, but how do I check for this. The fan does not come on for Heat Forced Air (roof mounted central air & heat) System. The fan does not work in either the "Auto" or "ON" position, & the HG switch is correct. HELP > It's getting cold :)

You have a combination fan/high limit that is screwed onto the heat exchanger. This needs replacing. Trace the fan wires and you will see it. It's rectangular and when the cover is pulled you will see a dial with sliding setting tabs on it showing the degrees it will turn on the fan. DrTom4444