I have been chasing this P0171 lean code for a couple months after a coupler change. First I thought it was a air leak but I couldnt find one, then I replaced the MAF screen and the light went from always one to coming on and off every other day. I was thinking I had to replace the gutted box but i didnt have time to do so. I went on a 300 mile trip and at the end of the trip the light went off and hasnt came back in a couple weeks when it use to come on every other day. I remembered that after the coupler/UIM change I disconnected the battery but never did the computer relarn cycle. Could that have been the reason the code kept coming back?