20002 Volkswagen Jetta with code P1128 and poor gas mileage

Problem:

Check engine light is on and when scanned, code P1128 came out which stands for lean upstream oxygen sensor. Gas mileage also got worst during this time. Do I change the oxygen sensor?

Basic Concept:

When this code shows up, check for vacuum leak in all vacuum hoses and top intake gasket. The oxygen sensor can be tested manually by feeding fuel or creating a vacuum leak. If the voltage reading of the sensor changes, it means the sensor is OK; otherwise replace the sensor.However, the most common fix for this code is to check and service the mass sensor. This will usually fix the problem.

Repair Technique:

Before doing anything, service the mass sensor first and reset the code. For details in servicing the mass sensor, contact ATS. If after road testing and the code still show up, then proceed in testing the vacuum leak and the oxygen sensor itself.
Details in testing the oxygen sensor is covered in ATS including the proper way to do a vacuum leak test.