Welding a catalytic converter in place is not a smog check failure. Most often, the failure occurs not for the weld but for a catlytic converter not approved for use on the vehicle being smog tested. When the smog technician verifies that the welded CAT is not the correct fit and design (not CARB approved) for the vehicle being administered a smog inspection, he/she must enter "tamper" in the smog machine for the visual inspection of the catalytic converter.
Per the California Air Resources Board, a catalytic converter can be welded on a vehicle if it (CAT) is ARB approved for use on the vehicle in question and if the installation weld is done professionally; as to not diminish emission reduction.
At times installers adjust the catalytic converter installation by using inlet and/or output adapters. A vehicle owner might run into a visual smog check failure (tamper) if an adapter was used in order to accommodate an ARB approved catalytic converter to a vehicle and the weld job was not done properly. The CAT and adapters must be a secure fit and the weld be clean; as close to factory as possible. No air should enter the CAT either before or after the welds. And most importantly, the oxygen sensor(s) location must not be touched.
