P0171 and P0174 codes for 2000 Hyundai XG300 with 3.0L engine

Question:

This 2000 Hyundai XG300 runs pretty good but 3 days ago, the check engine light showed up accompanied by codes P0171-Bank 1 system too lean and P0174-Bank2 system too lean. My vacuum reading looked normal and I did a quick tune up like replacing all filters and spark plugs. Since the oxygen sensors have not been replaced since the car was new, I replaced both the upstream sensors. The downstream sensors (after the CAT) are not used to monitor engine fuel mixture so I left them alone. However, after doing all of these, the light came back with the same code. Suggestions?

ATS Advisor:

The most common cause of this problem is a dirty or faulty mass sensor. You can service that by cleaning the sensor wire elements as per ATS course. If you see any improvement after servicing but the code returns after a few days. Replace that sensor too. In case this fix will not work, here are other areas to explore:

Are you using premium gasoline? It would help if you it and may also improve performance.
Have you done fuel chemical tune up? If the engine has too many miles, the injectors could build up varnish and ultimately carbon in the combustion chamber causing this lean condition.
Adding chemical additive to your fuel tank can help remedy this. however, if the code persists, have the injector serviced by a garage.