Originally Posted by pyroskier
I was there for a long time, I feel ya. I still get a few P0171's from time to time. Have you checked your injectors? What about your CAT? Mine most often comes on in the summer about twice a week, and in the winter about once every month. I just delete the code since I can't find anything. I have an aftermarket CAT so I think that might be doing it for me.
With an aftermarket cat there is a few things you MUST know. The distance from the stock honeycomb (loss of words brain fart) too the o2 sensor must be the same as the aftermarket. So if the stock is 4.5" to the rear o2 and the new one is 5.5" away you will have to cut some of your exhaust pipe to make it closer. This could be your issue not likely.
Make sure to check for vacuum leaks downstream of the MAF
Check condition of PCV over time heat and cold can cause them to become brittle and crack.
Monitor the o2 voltage it may have failed/faulty
Check your injectors one maybe stuck or plugged, you said you checked the filter but is it running ok when the lights on and off? Check FRP
And the most over looked False Air aka any exhaust leaks before the o2
Hope I could help