1996 Ford Aerostar P0171 Code Repeated - Check Engine Light On After MAF and O2 Sensor Replacement

I own a 1996 Ford Aerostar with a 4.0L V6 engine. I've been experiencing recurring P0171 trouble code — 'System Too Lean Bank 1, Sensor 1' — even after replacing both O2 sensors (one before and one after the catalytic converter). The mechanic advised that O2 sensors should be replaced every 60,000 miles, and my vehicle has reached 127,000 miles, so replacement was overdue. I also replaced the Mass Air Flow (MAF) sensor due to a noticeable humming noise. I ran injector cleaner through the system as well, but the P0171 code persists, and the check engine light remains on. The MAF sensor still produces a hum. I understand what 'System Too Lean' means and know several potential causes — including fuel delivery issues, vacuum leaks, or faulty injectors. However, I'm confused about the location of 'Sensor 1' in Bank 1, since neither O2 sensors nor the MAF sensor can distinguish between Bank 1 and Bank 2. I plan to replace the fuel filter (as it's overdue) and consider reconditioning or replacing the injectors. I also suspect issues with the fuel pump, pressure regulator, or individual injectors. Can anyone confirm where Sensor 1 on Bank 1 is located? Are there any other common causes for this persistent P0170/P0171 code in a 1996 Ford Aerostar? Any advice or troubleshooting steps would be greatly appreciated.