It is not necessary that you have the stock catalytic converter but it is required that the CAT is designed specifically for your 2003 Dodge Ram 5.9 Cummins. In other words, you may have installed an aftermarket catalytic converter so long as it is CARB (California Air Resource Board) approved for your vehicle and possess an executive order number, also known as an EO number. The EO number must be stamped on the CAT, and the smog technician will visually observe the EO number and ensure it matches the catalytic converter EO number for your truck. If the EO doesn't match, the smog technician must fail your Dodge Ram, indicating on the smog inspection report a "tamper".
