I'm going to the exhaust shop tomorrow because I hear a small rattle at idle (sounds around the cat, but also in one of the mufflers)
Perhaps there is a leak before the 02 sensor... but as i said , exhaust is brand new 2 weeks old..
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Cat was blown apart, so the removed it and put in a "test pipe" for the duration of my tuning. no other leaks detected.
unless something is expanding and letting in a lot of air when it gets really hot...