What else could cause a misfire if I have replace the cool pack. on 2005 Saturn Ion

Hooked the car up to the computer at the mechanics. Check engine light was on and exhaust had awful smell. Replaced spark plugs, boot and cool pack. Any suggestions.

Asked by for the 2005 Saturn Ion
Cat usually causes bad smell. Might need to check injectors. Switch 2 injectors see if misfire moves.