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I have a 94 base 3100 that had the O2 sensor replaced before I bought the car. It went bad and was replaced but the computer still thinks it's bad and trips the "Check Engine" light. I was told that the only way to fix this was to take it to a dealer and it'd cost some good money. Is there any way to get it done cheap?
It isn't a big problem but I'd like to eliminate it. How would I know if the light ever comes on and it's actually legit? Know what I mean? Thanks.

I believe if you take the negative terminal off your battery for 5-10 minutes this will reset the computer.  Although the person who changed the O2 sensor should have disconnected the battery while they were replacing it.