You're close, but H.E. furnaces dont have thermocouples, which require a constant pilot light. Studhauler needs to clean off his flame sensor, as scale will build up over time and impede its ability to tell the gas valve "steady flame, go ahead and open the main valve"First thing to try is replace the thermocouple if it has one. If it's dying it acts like what you describe. Also this is the simplest and cheapest thing it might be.
Studhauler, don't use anything more abrasive than steel wool, or else you can damage the flame sensor.
" My thought is; the furnace didn’t draft right with the cover off. Temps outside have been mild since I last had trouble. The next cold snap ( sun zero) will be the test. "
Incorrect. H.E. furnaces dont care where they get their combustion oxygen from for the burners. And your "draft" motor (inducer motor) has to spin up and move enough air thru the flue/exhaust pipe to close a pressure switch before it will allow the HSI (hot surface ignitor) to begin to light.
You don't have the furnace installed in a closet with a solid door do you? If so, you need a pipe going outside to bring in fresh air for the burners to run.
Curious, what diameter and pipe material is the gas supply line to your furnace? And what size supply tanks do you have?