2002 Buick Lesabre Service Engine soon light

My car has the "Service Engine Soon" light coming on and then going off. It will be off perhaps 20% of the time and on 80% of the time. Car runs fine.
I took it to my mechanic last week and he told me the oxygen sensor need replaced per the computer. He replaced it, and a half hour after leaving the shop, the light came back on. Took it back and he said he couldn't find the problem and he replace the new wensor with the old one and returned muy money.

I hate dealers with a pasion and they will just start replaceing things till the light goes out, not caring a bit about trhe cost to the owner.

Any ideas?