Originally Posted by andrewwynn

Theory; MAF reading low:

This should mean that the O₂ sensors will add an equivalent percent of fuel to make up for the error.

STFT ≈20%

3.5 MAF times 1.2*3.4=4.08

Add 7% long term 4.08*1.07=4.366

4.37 is right in line with a baseline error free engine.

Prime suspect: MAF.

You can get a $20 test case off Amazon I would buy a Siemens OE MAF once I was convinced.

(FYI; when my m54 was about 190,000 miles I did just that when my fuel trims were climbing steadily over time)

Not sure if airflow readings are affected but I would def check the condition of the air intake filter.