I have a 2005 Aveo, with about 86K miles on it. I had an O2 Sensor code (can't remember off the top of my head), replaced the upstream sensor, and code disappeared. Not long after replacing the sensor, I received a P0171 code (Engine Bank 1 Lean). So far, in troubleshooting, I'm not coming up with anything to indicate why - no apparent leaks (including going through lines with propane) and car runs great. I have not had the O2 sensor long enough to see a difference in fuel consumption, with the car at 29MPG in city conditions, both before and after replacement.
Potentially related is that when the vehicle is cold at first start, it takes a few moments of turning over to start - almost like fuel is returning to the tank and taking a moment to get back up there. After it starts, no issues to restart, unless left for a while. The longer left, the more time it takes to start. Additionally, when the car is at 1/4 tank, it hesitates, acts like it's losing fuel, and will stall out on sharp turns (like fuel is sloshing below the pickup point. When I fill it, condition goes away and car returns to normal operations.
I'm waiting on my fuel pressure tool to arrive (local shops want to charge the price of a tool, and the auto parts store's tools are broken), and that is my next check. I *suspect* that I have something wrong with the fuel pump area (pickup point, clogged screen, crack somewhere), but I hope not. That puppy is expensive!
Let me know what you guys think - or if I'm off my rocker.