gibson headers

[cas9832]

I just bought stainless Gibson headers for my2005 Crew Cab Sierra. Has anyone had any problems with the install/performance? Also, the dealership said that I might get a check engine light. I allready have a FPIK and flowmaster and this hasn't been the case. I was under the impression that I was in an "open loop". Why would this change by addding headers if the O2 sensor is out of the loop? I am going to put them in in a couple of days so any advice is appreciated.