I recently installed new plugs and wires on my 4.0l ranger. The check engine light has always been on and I have had it checked before. It always reads too rich or lean in bank one or two. I take it that this is for an o2 sensor. I had it checked again the other week and it read that I am misfiring on 4 out of the 6 cylinders. I figure if that were true that I would be in for an extremely rough ride, but my engine seems great. I am not noticing any misfiring. Also if it were misfiring that bad wouldn't I experience a substantial drop in my gas mileage? I am still at around 16 mpg around town. Any ideas on what the problem may be? Any help would be greatly appreciated. Thank you.

 

what kind of plugs did you put in?