Mazda not running after catalytic converter replacement

My husband,who is an engineer and has done all of our car maintenance for decades is stumped by our 2000 Mazda MPV. He replaced the catalytic converter and put every thing back together, but the car won’t run. Will start, but dies right away. Took off new cat,and runs with that off, but not sure why it won’t run with the cat on. Any thoughts?

Perhaps disconnect the battery and leave it disconnected for a while. Not sure on that particular Mazda but do know that some cars (ie Subarus) retain settings related to readings they’ve made over time. Since he’s swapped a part out, those settings are not valid anymore but adjustments based on them are still made. Of course, they are way out of the ballpark now.
Usually disconnecting the battery for a while (overnight, maybe) resets that history.

It is worth a try until someone here comes up with a better idea.

The new part may be blocked. This problem may be worth looking into…and removing whatever is in it.

I agree with the above on the new part having a blockage, I belive you should be able to see light through it from one end to the other if all us clear.

Was this a “new” converter or a used one. A used one could be plugged up.