Looking for advice/ideas on if theres a common problem with both my codes or just what you think they might be individually.

I'm showing: P0420 Catalyst efficiency below threshold (Bank 1)
and P0171 System too lean (Bank 1)

I just replaced my 02 sensor 3 months ago and I don't feel any any misfires. Please help.

2002 A4 1.8T

 

Check for vacuum leaks and reset the CEL.
If P0420 comes back you need a catalyst.