Does this look like a crack in my heat exchanger?
I had a similar experience. I did not trust the tech's report and felt like I was being setup for a replacement instead of a repair.

The tech's report mentioned a smoke test. More research online revealed a match test. My furnace has flame injectors. Remove cover to injectors. Turn on heat. Observe flame. Any orange? Even intermittent? Possible crack. Turn off heat. Turn on fan (only). Turn off gas at valve by furnace. Ventilate to eliminate gas traces. Light a match and hold it up to each heat exchanger opening. Does the flame move as though air is passing over it? If so, exchanger is cracked and should be replaced.

I found a new exchanger for my 19 yo Goodman 92% furnace online for $100 + $40 shipping. Used Permatex Red to replace gasket to secondary heat exchanger.  It was quite an effort.

(Side note: had to replace control board before I could perform above test.)

All summed up: $80 tech visit, $75 board, $140 heat exchanger, $20 supplies, gas sniffer (forgot how much - worth peace of mind - good tool for any DIY'r)
Experience: priceless; when I finally replace this furnace I will do it myself
Alternative: $800+ new furnace (DIY) or $$$$$$$ by HVAC Inc.