slowness

So I recently put on a MF race 3" muffler and dumped it at the axle. Its a 2007 NNBS with the 4.8 with S&B intake and that's it. I love the sound and have had no problems till about 4 days later ,now it thrown the P0171 Bank 1 lean code. Any Ideas??

FFDP

iTrader: (2)

It's not muffler related.

Check for exhaust leaks pre o2 sensor (upstream) and intake leaks/vacuum leaks.

slowness

So I figured it wasn't bc of the muffler, but nothing else has changed except that. I remember reading something about the nnbs throwing codes after replacing muffler. ill have to throw a smoke machine on it tomorrow. there are no exhaust leaks. Has anyone ever figured out the notorious slight hesitation while stopped and in gear??