Looking at the wiring diagram, it seems clear to me that the fan is controlled by the G and Y terminals on the control box, which is controlled by the G and Y terminals on the thermostat. Which are probably connected by green and yellow wires. And maybe (unlikely) the thermostat is bad, or stray wires from the G or Y are touching the other terminal. And at the very least disconnecting one of those two wires will stop the fan or not, and give essential debugging information . (And reconnnecting it right after, because the fan is needed when the burner is burning. I'm talking about the warm air house circulation fan, not the fan that blows air into the oil burner.)
Also, and this I'm less sure of, the fan can stay on for months without stopping and without burning out without shortening its life much. So being on 24/day for a few days didn't hurt the fan, but might have cost him a little money.
I don't need any help on this afaik,since I've spent plenty of time on an identical furnace.... unless I said something incorrect above. Did I?
**If I hadnt' had bad experiences with him, I'd print out the wiring diagrams of the furnace etc. and take it with me tomorrow evening, but he's been such a pain. He never identifies himself when he calls. Last winter when I went over and couldn't fix it, I asked him when he got it fixed to find out what the problem was and let me know. He didn't do that. I asked him to call me when the paid repairman is there. He didn't do that.
But the prize was when he wanted an oil pump, and I have spare parts for many things because when a neighbor got a new furnace, I got the old burner. So it was the day of the funeral of one of my best friends (and he knew that), my first good friend to die, under depressing circumstances, and after I got home from the funeral, instead of going to sleep like I wanted to, I took the time to take the oil pump out of the burner, clean up the dripping oil, wrap it up and put it on the porch, as I told him the night before I would do. He never came to pick it up, he never called to tell me I could put it back in the house, he never called at all, and he never thanked me. Nothin'.
Since then I've brushed him off a couple times, and I wouldn't even go tomorrow except I'm curious and he lives less than 100 yards away.
I'm especially curious because he told me that pros told him the control box needed replacing. (He called me 3 or 4 days ago to get my spare control box but a) I used it already, b) he told me the problem and I told him the control box has nothing to do with it.) He gave me the name of the company that told him that and their webpage says they've been in business since 1983. That's a long time to not understand what controls the fan. His business address seems to be his home, and he has a biggg home, 2 2/2's or 3 1/2 high, with a biggg swimming pool, and a tennis court, a 3-acre lot backed by acres of woods, neatly trimmed bushes and neatly mown lawn, in an expensive suburban n'hood not so far away. He couldn't screw up that often, so this must be some shlock employee giving out bad advice.