A catalytic converter is required on most motor vehicles to meet federal and state smog requirements. And, catalytic converters (CAT) to need replacement. When replacing a catalytic converter, be aware that each CAT has a stamping/identification particular to that engine and class of vehicles. The smog tech doing the test is required to verify that the CAT number is an exact match to the emission family number listed on the under hood label.
The orientation of the number on the CAT also has to be were the smog tech can read it. Worst case, the CAT needs to be cut out and repositioned so that the smog tech has a clear view of the tag number. This step makes use of a generic (low cost) CAT impossible. To ensure compliance, the California Bureau of Automotive Repair does send test vehicles that have small things not to the letter of the law to catch smog techs that are not complying with the law with stiff fines for non-compliance.