2002 Monte Carlo SS has lean code....HELP!!

We have a 2002 Monte Carlo SS with a 3.8L. We have got a lean code that we cant get to go away. This is the second motor because the original one locked up. The code has existed with both motors. We have replaced almost everything. My brother in law is a mechanic and this even has him baffled. He is the one doing all the work so i figured i would see if anyone else has had this issue. He has replaced intake gaskets, computer, fuel injectors, throttle body gaskets, checked for vacuum issues, O2 sensors, and everything is still the same. Can anyone give us some more ideas on what to check and or replace. Have to get it fixed to pass emissions and we cant get it. Any help and ideas would be greatly appreciated. Thank you all in advance!

Welcome to AF, what is the exact code are there any other symptoms? Checked fuel pressure? How and when does it set a code?

Welcome to AF, what is the exact code are there any other symptoms? Checked fuel pressure? How and when does it set a code?

Thanks! The code is 171. There are no other symptoms. The car drives fine despite the check engine light being on. The fuel pressure was checked and was within specs. After resetting the code it usually takes about 10 miles for the light to come back on. Thanks for responding and i hope to hear back from you again.

Check for any vacuum leaks or air getting in behind the MAF, also have you recently replaced the pcv or intake gaskets?