So long as the CAT you used was California certified (50-State Emission Legal) you can pass the smog check with it bolted and flanged on. Having the Catalytic Convertor welded on is not a requirement unless there is a noticable leak where it was installed.
As standard practice, smog stations like to see an old CAT which was cut off and replaced with a new one, welded on. And CAT removed simply by lossening bolts, bolted back on.
posted by SmogTips Support