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I go by the age and performance of the car. If the cat is like 90k miles old and I have doubts about it, I'll change it.
Sometimes they will leak out the sides if they have a significant clog. Have someone place a rag over the exhaust while you listen by the cat. If it hisses out the seams, you have a clog.
Where I live, we have emissions testing on anything 25 years or younger. My '84 was flunking and I put a new cat on it last year. The emissions are great now and it has more low end power.
Bob
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