P0171 Lean Bank 1 Fault on 2008 VW Polo 1.2 6V - Diagnosis and Solutions

Hi everyone, I'm currently troubleshooting a recurring P0171 'Lean in Bank 1' fault on my 2008 Volkswagen Polo with a 1.2L 6-volt engine. The check engine light (EMR) activates after the engine reaches operating temperature, typically around 10 minutes of driving—usually on a motorway with steady revs at approximately 840 RPM. I don't experience any noticeable symptoms such as misfires, rough idle, exhaust smell, or power loss. I've already conducted an air leak test using propane and haven't detected any leaks. The engine runs smoothly under normal conditions, and the O2 sensor has been replaced without resolving the issue. Using a diagnostic reader, I checked key parameters: the MAP sensor reads atmospheric pressure when off and drops to about 30 kPa at idle. I cleaned the MAP sensor, which reduced the LTFT from 14.4 to 12.5 (STFT is 0), but the P0171 code persists. The O2B1S1 voltage fluctuates between 0.060V and 0.780V—within the expected range of 0.1–0.8V, with an average of 0.42V (close to the ideal 0.45V). I've confirmed that even after removing the MAP sensor, the engine continues to idle within this voltage range. When revving the engine, the O2 voltage briefly exceeds 0.9V but returns to the low end when RPM stabilizes at high levels. Possible causes I'm considering: 1) Undetected air leak (possibly in intake or vacuum lines) 2) Clogged fuel filter restricting fuel flow 3) Damaged wiring harness leading to O2 sensor malfunction 4) MAP sensor calibration loss due to improper operation or cleaning I have not yet tested the power supply voltage to the O2 sensor, as a 60mV drop seems unlikely to cause a fault—but I'm open to suggestions. My current suspicion leans toward the MAP sensor losing calibration. However, the fact that the car runs with low O2 voltage even without a MAP sensor is confusing and raises doubts about this theory. Any insights or experiences from owners of similar VW Polo models? Thanks in advance!