Yes you will keep throwin codes unless you get a o2 sim or you get your pcm tuned to read no report for the o2 sensor. I cant imagine it hurtin your mpg that bad tho, but someone correct me if im wrong but i thought the plugs were suppose to be gapped to .055-.060 for stock plugs. And depends on how your driving it, if your flooring from one redlight to the next then you should expect mpg like that. I have a 98 with no cat, egr, or o2 and i still get 31 on the interstate and about 23 in town