Check engine light code 174 lean bank 2. Was told it was intake plenum gasket on 2004 Ford Taurus

Have a check engine code 174 engine lean in bank 2. was told to replace intake plenum gasket. I did, and throttle positioning sensor,imur gasket, idle control valve cleaned, mas airflow sensor cleaned.
Why do I still have this code. Could it be a bad o2 sensor? Oh I also checked all vacuum lines for leaks.

Asked by for the 2004 Ford Taurus
More than one engine was used in these cars you know which one you have??
I have a v6 OHC Engine with 131,000 miles on it.
3.0 DOHC V6. Did you replace the INTAKE gaskets? If not that may be the problem. Could have it tested.
Good luck with it.
OHV V6 I replaced the upper plenum gasket because the engine was running pretty rough at idle and I was getting the code so I was told to replace the upper intake plenum gasket so I did. I even replace the runner control gasket, cleaned mas airflow sensor, cleaned ide air control valve, and checked for vacuum leaks on all hoses and never found any leaks. Since I replaced the upper plenum gasket the car does run alot smoother but the code came back. When I say came back, I disconnected the battery to remove the plenum since I had to unplug all the electrical components that were connected to the upper plenum.