Gas and Oil Home Heating Furnaces - Carrier Weathermaker SX Continued Problems

I have a Carrier Weathermaker SX, Model #58SX100-CC-1, Series 170, Nat Gas, from 1987 (but used rarely - only to maintain 50 degrees in winter to keep vacant lakehouse from freezing). This is the 2nd time I ha ve had this problem.

I tested the unti when I opened up the house this summer by turning up the thermostat. I never heard the heat come on, but when I turned the thermost down and turned the setting to OFF, the fan kept running and nothing other than pulling th fuse will shut it off. I believe it is the "pre-fan" for fresh air intake that I am hearing running. I checked the electronic ignititer "glow-plug" and that is not coming on.

Do I start by replacing the thermocouple ignitor? Or is there something else to check first? And is there anyway an applicance furnace repair shop can test the ignitor outside of the furnace if I bring it in to them (since I have it with me and no longer installed. Any help?

fan on all the time could be open limit, open safety circuit (check resets on roll out switches)

older carriers ran the fan when transformer was bad

does the unit have a circuit board? it may have a diagnostic light

just a few thoughts