My boiler (circa 1985) "eats" thermocouples, cheap ones last six months, better ones last a year or two. They were <$10 each pre COVID (the "better" Honeywell ones), $13.99 now. I just keep a spare and when it fails I swap it and I buy a new one and hang it beside the boiler. The first time swapping it out took some time, now I am down to about 10 minutes. Big time savers, I marked the flare wrench I need for the job with tape and figured out how I can access with minimal disassembly. When I had the boiler last serviced he made some adjustments but it did not make a difference, for the cost per year I just swap them now....
It could be deep in the furnace but it is such a cheap part I would consider swapping it if you are comfortable working on it yourself, very small investment that may save some big money or differ it. They are also technically a consumable so I bet it is not as hard as he made it out to be, and the "firebox" where it is located should be "accessible" for other service as well.... For mine, it is the same form factor as a water heater one so they are super easy to buy....