I have a borla exaughst and dc headers, unknown Catalytic Converter. the guy who owned it before had the check engine light come on so he took it in and the Catalytic Converter was messed up and ruined the o2 sensor so he had them "fix it" about a month after i bought it check engine light came back on i took it to my own mechanic and they said it was the same thing but it didn't matter that it wouldn't hurt my car and all that it would do is turn on the check engine light, is this true? is there a certain cat that i can get that wouldn't do this?