I have done a lot of searching for help on this error code, but most topics point back to some sort of aftermarket intake being the culprit.
I had a Stage 1 Cobb AP map, and a K&N air filter. Car has run awesome until just this past Monday. It started bucking really bad at low speed (in the parking garage), and turned into more of a hesitation when I got on the highway. Sure enough, the CEL turned on and I read out the "P0171 System Too Lean (Bank 1)" code.
I immediately took the K&N filter out and replaced with a standard one (I know this wasn't the culprit), and also un-installed the accessport so I could return to stock settings. This didn't help. The car feels like it is going to stall at idle.
I filled up with gas the night before all of this happened, but I have never had problems with this particular station before. I have now put some dry gas in the car, and topp'd off with 93 octane from another station.