If you search for “2005 Nissan Armada service manual” , you should have no trouble. You can try “x.nissanhelp.com” for starters.
You must have the “pathfinder armada” version. The 2005 manual will be just fine.
As for the code,
You can go so any autozone and they should be able to help you pull the code. You can also do it yourself if you have the obd reader (get one on amazon) and free software from your smartphone’s App Store.
Could be a faulty MAF and could be the idle air. You can reset the ECM (you can find those instructions online or on this site) yourself with the key on/off accelerator press procedures.
Sounds like you went to a 3rde party mechanic. Amazed the mechanic didnt give you the code!?
Did you just get this car? Have you had it since birth. Or is it a used car you bought? Since I don’t know the history of your car , it’s difficult for people on the internet to give specific help, but your issue sounds like its easy to replace, once you can get the codes.
Could also be other issues (CATs, O2 sectors, etc...)