I have a P0420 code (insufficient cat efficiency) and am trying to isolate the problem. Car idles, drives fine. I do hear very slight emission backfire at 40-50 mph, but hardly can hear it over the radio. I replaced wires, plugs, coil pack recently, and I'm gonna check the injectors tomorrow (with ohmmeter). I think the fuel mix may be a tad rich. Any other suggestions? From what I've read, the O2 sensor is usually not an issue.