Thank you everyone for the help. I thought i'd provide an update.
I changed the cap and reset the computer by disconnecting the battery (next time i'll try the fuses). So far so good, no CEL, but i did't drive that far. I'll update again after some miles.
The cap now goes on a lot easier. I don't have to fight with it to get it on. I wonder if the old cap has an issue with heat. It started to become a pain last summer, subsided in the fall and became a real pain this summer.
It would wonderful if this new cap fixes the problem. Regardless, i will certainly attempt more repairs myself.
