A code 420 is one of the most difficult and annoying trouble codes in any car system and can literally be caused by hundreds of different things. O2 sensors "sometimes" are the cause, but from my experience its not common. The only thing rear of the secondary O2 sensors that could cause a code or running problem is a blocked exhaust. If your invidia muffler is old and the glass packing inside comes apart, it can cause a restriction. Is this what you mean by it got burnt out?
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