I have a 2000 with the 5.4 that's throwing a P0171 code. Lean on bank 1. What could cause only bank 1 to run lean? If anyone else has had this, what did you do to fix it? Any advice is greatly appreciated.

 

vac leak, bad injector etc

 

I cleaned my MAF valve, changed the air filter, installed a new PCV valve, and ran a bottle of fuel injector cleaner through a tank of gas. After that, I changed the fuel filter.

That was the easy and cheap maintenance and it fixed my problem. You could however, have a vacuum leak elsewhere that is causing your problem