Could it be it is running only on one element because it is not cold enough outside. It is a feature of this furnace. I overcame this issue by rewiring thermostat so both elements start together. Heats faster and runs for a far shorter period

There are more than one set of electric coils in these units. You can test each one with an amprobe to see which one has no flow. Than it is a matter of finding out why. Sequencers stagger the starting order of coils in electric furnaces, could have one of several other issues!