01 Blazer-code P0155 and P0141 after lower intake gasket changed?

I have a 2001 Blazer LT 2wd with the 4.3 vortec (W). I changed the lower intake gaskets and valve cover gaskets (thanks to this forum). The check engine light came on right after I changed the gaskets with codes P0155 and P0141 (oxygen sensor heater circuit). Could this be caused by anything that I did when I changed the gaskets? Other than the check engine light the Blazer runs great. Any help would be appreciated.