About 2 weeks ago my car set a P1131 indicating an upstream O2 sensor lean condition. I hadn't changed the spark plugs on the car for 60k miles so I bought a new set of plugs, threw them in, cleared the code, and hoped that would be it. On the positive side, the plugs absolutely needed to be replaced because the gaps on two of them were awful. On the negative side, it didn't fix the problem. Since the initial SES code, I've had two incidents were the car ran badly. Once at a stop light the car was idling very rough and only picked back up once I gave it some throttle. Another time I was accelerating up a small hill from a stop and at around 20MPH I began to lose power. After about 5 seconds the car picked back up and drove normally. In both these cases, the car was still in open loop and had only been running for 3-4 minutes tops. Other than these two incidents, I don't really feel like the car is running badly. Fuel mileage seems normal, idle is steady, power feels normal. Driving home last night my car threw the SES again with P1131. Unfortunately, my scan tool does not have a live data feed, so I can't watch the O2 sensor voltage in real time. All I have the single code and the freeze frame data. Code - P1131 Lack of HO2S11 Switches - Sensor Indicates Lean Fuel SYS1 - OL-Fault Fuel SYS2 - NA Calc Load(%) - 45.9 ECT(*F) - 150.8 STFT B1(%) - 0.8 LTFT B1(%) - 0.0 RPM: 1826 Veh Speed(MPH): 27 The fault being in open loop makes me think this has nothing to do the O2 sensor itself since the O2 sensor isn't controlling the fueling in open loop. I've checked for vacuum leaks and didn't find anything or hear anything. What should my next steps be?