If you have an android phone, check out the Torq app and the ELM327 bluetooth OBDII dongle. Works pretty good as a cheapo code reader, to get at least the generic OBDII failure codes. Failing that, I think several auto parts places will pull codes for free. Or ask someone at work--I've pulled codes for coworkers.
If it's an evap code I don't see it changing how it runs. EGR, it might, depends upon the engine family. Cat efficiency, nope. Probably several other categories that won't affect mpg.