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Happy Wednesday All!
I've had a P0421 coming up for a while now, and with the cold weather we've been having, I've noticed a lot of noise on cold startup. It almost sounds like the exhaust pipe is UNDER the hood. I also get a strong smell of exhaust gases in the passenger cabin if I turn on the blower fan. (much stronger than normal)

Here's the weird part, if I put the car in reverse, the sound disappears completely. I'll try to get video/audio later today and post.

A few months back, I had a bad rear cat on bank 1, I'm wondering if the back pressure blew out the gasket on the precat and there's just a little gap there that is letting this all escape and causing the 421 code because not all of the gas is travelling as it should down the pipes.

Any input on what to check for, how to replace the gasket is welcomed.
The precat is only two years old, so I'm pretty confident it hasn't failed but I won't rule it out.

Thanks in advance!
 

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I have similar symptoms. Most likely due to an exhaust leak. Eliminate exhaust leaks and see if it goes away before considering replacing the rear pre-cat.
 
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