We won't be able to tell exactly why the CAT was considered modified without
seeing it. However, common modifications are plugged AIR injection ports on
a Three-Way CAT (which your truck should be equipped with), also if the CAT
which was used is not an original equipment manufacturer (OEM) part this too
would cause a modification failure. Mind you this doesn't mean you have to
buy the CAT from the dealership; OEM parts are widely produced by
aftermarket emissions parts companies. The important thing is to ensure the
"right" OEM Catalytic Converter was installed on your Dodge Dakota.
posted by SmogTips Support