OBD2 Code P1131 on 2002 Ford Taurus Vulcan 3.0L V6: O2 Sensor Location & Diagnosis Guide

I'm currently troubleshooting a persistent OBD2 fault code P1131 in my girlfriend's 2002 Ford Taurus with the Vulcan 3.0L V6 engine, which has accumulated 122,000 miles. The vehicle is showing Code P1131, and I'm trying to understand whether this relates to a specific oxygen sensor location or if there are other underlying issues. First, I need clarification: Is the 'Bank 1' sensor located on the firewall side (radiator side) or the back side of the engine? The car has four O2 sensors — is the P1131 associated with an upstream sensor between the exhaust manifold and catalytic converter, or downstream after the cat? I suspect this may not be the only issue affecting performance. The vehicle currently idles roughly (slightly), and fuel economy has dropped significantly to around 12 MPG — far below the expected 18+ MPG. While it runs otherwise fine, I'm concerned about whether a failing oxygen sensor or other system issues are at play. I have limited access right now due to a broken leg, but I plan to address this soon. Can anyone confirm if P1131 is typically caused by a worn-out upstream O2 sensor? Are there any common contributing factors such as vacuum leaks or engine misfires that could trigger this code? Any insights on sensor accessibility and repair difficulty would be greatly appreciated.