At this point, I had three suspects, the transformer, the air proving switch, and after closer inspection, I saw a brown discoloration on one of the high voltage capacitors.

I decided to start with the transformer. After removing it from the unit, I checked the secondary and measured about 7 Ohm, reasonable. The primary was open, just to make sure I slowly opened the primary coil and after getting back to the winding, didn't see anything obvious, but it was still measuring open.

On removing the air proving switch, the thermistor checked out at about 1kOhms and responded with a squirt of canned air. All the discretes checked out except for the triac.