It would be helpful to know how they reached these conclusions, and if there are other codes besides P0420.
When I googled p0420 these are some of the potential causes that come up, so I think thatās where. At first I thought they were telling me that these were all problems my car has because they werenāt very clear but I donāt think thatās the case. I believe the 0420 code is the only one logged.
IMHO the OP should tell the mechanic everything he/she knows about the symptoms and the vehicle, and show him the receipts he/she has. That will also help him look up the part numbers used and confirm that wring parts arenāt the source of the problem.
[quote=ādb4690, post:15, topic:96567ā] Another thought occurred to me . . . if incorrect parts were installed, all bets are off. For example, if this is a California emissions car, and 49-state components were installed, it might get interesting
[/quote]
Thatās good input too.
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an exhaust leak can cause a P0420 code, and you know you have an exhaust leak.
Start there.
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I think you might be on to something with the incorrect parts theory. I updated the main post with what I think is going on.
In many states, you fail inspection if the check engine light is onā¦
my state doesnāt have inspections 
Solved: the catalytic converter was defective. We took it to a repair shop and they couldnāt find anything wrong, so they took our receipt and went to the car parts store and got a replacement cat under warranty. $130 for labor to install it and the car is good to go, no check engine light in over 2 months.
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Iām glad you got it fixed. I bet the dealer it originally came from knew about the issue and did a wholesale to a mom and pop corner dealer. I have seen traded cars with mechanical problems sold off to small corner dealers because the big reputable places donāt want to deal with them.