If you found evidence that your header gasket was leaking, would you take the time (and risk of shearing bolt off in the engine block) to replace the gasket?
My car has been throwing the P0420 with increasing frequency, and has high fuel trims (+8 to 12% at idle air loads). I suspect that leaky exhaust may make the A/F sensor read lean, so the car adds fuel, leading to too much carbon for the catalytic converter to handle. (does this explanation make sense given my symptoms?)
On the other hand, my "evidence" that I had a gasket leak turned out to be a false alarm. Perhaps I was too fast to judge the black crud on the head as exhaust blow-by around the gasket.
When I pulled off the heat shields and exhaust manifold I saw the dirty spots, but there was no smoking gun:
No obvious blow by on the gaskets, manifold or on the heads.
Then, I made a test run for about 20 minutes to see if everything worked well.
My boost spools just as fast as it did before (thus I expect that I don't have a new massive exhaust leak).
My fuel trims seem to have changed very slightly after the gasket change, but I will have to see if this is permanent: I suspect that it is within normal variation. This is, however, the first time since I've been checking fuel trims that my numbers are so close to the acceptable +-5% range.
^ This, definitely fix it...it's an easy fix and you should have no trouble getting the bolts off. Just hit it with some good penetrating oil and let it soak in, you'll be fine.
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