I just changes my O2 sensor and I was wondering am I supposed to do anything to reset it or once I hook it up its good to go. And Another ? I have is what could be leading to a loos in fuel mileage becuased I'm only getting around 250-300 miles per full tank which is very low.

 

You don't have to reset it no. I would b/c it clears the fuel trims out & you don't have to wait for them to change on their own.
btw how many gallons are you putting in the tank when you fill up?


Maybe time to take the injectors out.