P1128 code for 1999 Jetta with 2.0L engine

Question:

I got this code P1128 -long term fuel trim lean that keeps coming back even after I reset the code. An inspection and voltage measurement of the oxygen sensor looked OK and there is no vacuum leak detected. The fuel filter was also replaced but the code remained. What do I look for to fix this code?

ATS Advisor:

The most common problem for this code is the mass sensor if you believe it! Located in the air cleaner ducting, its worst enemy is poorly maintained air filter that collects dirt and contaminates the mass sensor wire elements. See ATS past newsletters for corrective action.

Repair Technique:

When checking mass sensor, check also for signs of oil contamination in the mass sensor elements that can easily corrupt the mass sensor signal. If possible, put a new one since 9 out of ten calls for the mass sensor replacement.