Header Installed ?

I just installed a new header, because my old header had some big cracks.
I now have the check engine light coming on once the jeep has ran for about 3-5 mins. O2 sensor? What else could it be? All plugs are plugged correctly. Please help need to fix asap.

Probably. Did you perchance disconnect the knock or engine temp. sensors underneath. Oh, sheesh, now that I think about it, could be a number of sensors you may have inadvertantly disconnected. Suggest getting a can of contact cleaner and some dielectric grease and go over them.

Any symtoms other than the light?

All have been re connected.

No one has any ideas? Please help.