I have been battling a P0420 code for a while now, and by "battling" I mean that I've been contantly clearing it out... Well this morning when I scanned, it was accompanied by a P0171 code. Telling me that my system is running too lean.
Now, when I had my car inspected this year, my mechanic told me that the K&N filter I cramed into my fender would eventually cause the car to run lean just by itself. He basically told me to take it off and put the stock one back on. Of course I did not do that...
I'm not a professional mechanic by any means but I didn't think that alone could cause it to run lean, at least not enough to throw me a code. I thought that the PCM would "learn" and adjust the fuel mixture automatically. I think it maybe has to do with whatever is causing the p0420 plus a dirty filter or MAF. Peeked at the filter today and it is FILTHY.
I don't know? Could just putting a cone filter in your fender could cause the car to run lean?