Exhaust pipe

2005 Honda Element. Check engine light had been on, I put the code reader in it and it says it’s the O2 sensor, also the exhaust pipe broke. My idea was to take it to a muffler shop and have them weld it. I did that today, he put a code reader on it and it wasn’t reading the o2 sensor. He thinks cause my old mechanic already put a spacer on it so I could pass inspection. The guy said I can fix back here and the catalytic converter could be the problem. So I said ok put a new one in after market, he did that, fixed the rest of the pipe, did the codes and nothing came up and check engine light was off. I drove home and all of a sudden when I get home the check engine light comes on, I’m like what the helI went to the local auto parts store to use a reader but there was broke. Anyone have any idea what’s wrong with it? I’m pissed I said to fix the cat now.