I throw the check engine light about two weeks ago after I installed dc jdm header and carsound cat. Ran the code and it said bad cat. Yesterday the light went off by itself. Did it reset itself?

 

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Mine was bad on my '00 and it didn't reset itself. I put the stock one on before I sold it and the CEL was gone. Go figure....

 

As it was explained to me, the ECU evaluates a cycle of events to determine whether sometime is wrong. Since the carsound cat is not as efficient as the stock cat, sometimes it passes the ECU's test, and other times, it doesn't. I also have an aftermarket header and carsound cat and my CEL has been going on and off without any intervention (ie. resetting ECU) for the past couple years.