need guidance 02 w 1.8t CEL P0171 & P0172 CODES
 

Back ground:

2002 1.8t engine 157k miles

I have replaced the precat o2sensor and the MAS.
I have looked for vac leaks none found.

o2 sensor replaced 2 days ago. After c/o reset computer, CEL came back on after 90 miles.

How can both codes be present when code P0171 says system too lean bank one and PO172 says system too rich bank one? Can anyone advise?

thanks in advance!!