There is a "routine" that the computer goes through and checks everything... I failed my emissions and went and cleaned my egr , removed my battery for 10 minutes to "reset" the light, then went back, light was off but the computer kept saying it was not ready.... The guy gave me a sheet of funky driving instructions to allow it to go through everything at different rpm and speeds.... drove for 12 minutes and went back and it passed.... I got this info from a jiffy lube safety/ emissions guy...
Hope this makes sense and helps....