Just an FYI: In the mean time (York still close to dead) my Trane XE80 failed again over at the condo. LED on the control board flashes 4 times indicating a Hi Limit switch open. 3 hi limit switches are all connected in series with yellow wires. Blower comes on but it will never set the igniter to glow or light the gas.
There are 2 in the blower compartment, one on each side of the burners. These look more like a silver discs, again with the 2 yellow wires in series.
There's also one on the bulkhead wall that looks like a flat piece of fiberglass with 2 wires on spade clips coming out of it (long probe goes into the firebox). Look on the wiring diagram, the are all usually wired in series. Short out (jumper across) each, one at a time, to figure out which one is bad. Placing a jumper across my bulkhead sensor allowed complete furnace functionality to be restored. This part has gone bad twice in less than 2 years.
Note: Since the same part failed again, the long probe that goes into firebox, is there a bad lot (code)? This part has not even been in this furnace for more than one year.