D & C Mufflers and Brakes D & C Mufflers and Brakes Charged me for repairs they never did Joliet IL

I had the check engine light come on in my car so i took it to this mechanic shop based on a reccomendation. I take the car and before they even run the codes on the car, which I had done before i took the car there at auto zone, they say it's the catalytic converter. I left my car there for 3 days, picked it up on a Friday mid afternoon and by Tuesday morning the check engine light had come on again.

This shop tried to give me a receipt that didn't have any of the business information on it so I requested it and made sure they put everything they did on there. They showed me car parts when I requested but how would I know it came specifically from my car if I was not there when the work was done.

I ran the codes at the auto zone again and it was the same code as the first time. I took it to the dealership this time around and was told the catalytic converter was falling apart. How is even possible for a "new" catalytic converter to be falling apart if it had been replaced less than 2 weeks ago? The dealership was doing the work through warranty so I wasn't making them any profit. I asked about the work done at the other shop and was told only the Flex pipe was replaced, the car still had the original catalytic converter.

I went to request a refund and the mechanic that worked on my car, Tony, swore up and down it was a new converter and that to leave my car for a day and he would replace it. 1. Why would he need to replace it if he swore up and down it was new? and 2. Why would I leave my car with a shady mechanic after it had been confirmed by the DEALERSHIP that the work wasb't done the first time?