GM-GMC: chevy silverado oxygen senors, chevy silverado, oxygen sensors

Question
I have a Chevy Silverado 01, i came to a problem by code saying to change oxygen sensors so i did... now i have a code saying "System to Lean" how in the world do you fix it? im at my last string with this truck please help me. thank you.

Answer
In order to answer this question fully, I would need to know...
1)What engine you are dealing with 4.8/5.3/4.3?  
2)What was the original DTC
3)Which of the four oxygen sensors you replaced
However, generally speaking.  It is not uncommon for the 4.8/5.3 engine trucks to consume oxygen sensors in numbers.  What I mean by that is I have replaced 3 at once or 2 at once many times.  Assuming that your truck does not have a vaccum leak or any exhaust leaks, you may need to replace the other oxygen sensor(s).