I think anything I do with vinegar or whatever will just get burnt off.
If it is that green oxide that link says it takes a long time to form (without any ammonia). To get the fridge working I just scrape the two probes a bit with a screwdriver, but most of the ends stay coated. A good clean would mean I shouldn't need to scrape them for a good while I hope.
My plan is to take them out, clean them up, and put them back in. I will use acid first to see what effect that has. I will try for before and after photos.
I expect to have the fridge working right by next year.
