I'm recently replace the engine in a 93 p-u with a used engine (100,000mile).
My check engine light is on, so I did some research and did the jumper wire in the diagnostic box and got a two digit number, 21. Which has to do with the O2 sensor.

I'm not sure if the light was on previous to replacing the engine, since I did not own or run it.

I have been checking my gas milage and I'm getting 17mpg (city/highway), which doesn't seem to be off the norm from what I read. I would like to get more MPG if possible. I have a highway trip coming up and plan on checking highway mpg.

Should I have a shop do a diagnostic check to make sure I need a O2 sensor before I go replace it. If I do replace it, will the check engine light go off if that was the problem or will the shop need to clear it off?

Thanks

 

There isn't a real diagnostic check that I know about for an 02 sensor, unless it's totally dead...
I'd replace it yourself, it should clear the code.