P0171 code has been showing up (System too Lean)

For a little over a month, P0171 (System to Lean) code comes up. My brother has cleaned the throttle and the MAF. It then worked fine for a few weeks, then the code came up again and the sputtering started. He took it to O'Reilys Auto Parts. They had said since you removed the throttle and put it back in, it would need to retrain itself. He then removed it again and cleaned out other parts in the car. Just the other day, my car started sputtering again in the morning. It runs fine the rest of the day. Also, when I was out of town the other day, my car sat for about four days with a low tank of gas. When we were traveling on the highway, my reduced power engine light came on. Once I filled up with gas it was fine. I am just trying to see what could be the problem. Do I need a new O2 sensor, a change of the manifold gasket, fuel injector cleaning? Not sure. The car is good to drive, but would like to fix this problem.