I just bought a 2005 GMC Sierra 1500 with the 5.3 liter and 31k miles. Yesterday I hauled my camper about 60 miles to the dealership that I bought it at for some service work. While I was accelerating up a hill, the check engine light came on and flashed 6 or so times. I let off the gas and it went off. When I finished dropping the camper off, I was pulling back out onto the road and the engine stumbled and the light came back on steady. From there on out, when I pulled up to a red light and stopped, it would almost die. It did fine on the highway though so I went on home. I went by Autozone and they scanned it pulling the P0131 and P0171 codes indicating a lean reading on Bank 1, Sensor 1. I don't want to overreact and change out the o2 sensor as it may not be that so any feedback from you guys would be great. We also have a 2004 Tahoe with the 5.3 liter and I have noticed that my Sierra does not seem to have as much power as the Tahoe but I assumed that had more to do with the 3.73 rearend in the Tahoe and the 3.42 in the Sierra. Don't know if this would indicate anything. Thanks for the help!