It was a clogged cat.:willy_nilly:

:party::cheers2::party::hurray::hurray::party::wil ly_nilly::hurray:

Cool, congrats on the diagnosis!

Afraid I have seen plenty of them. Was the classic description by your write up and video confirmed it.
When you pull it, check to see if there is any substraight left, if not check to make sure the screen mesh is still in the cat. The substraights are wrapped in a stainless screen, many times when the cats clear out completely that screen gets blown out. Usually they will get caught at the exit. Some times they do not and get blown into the silencers. (getting them out can be a challange)
This will also happen with the multiple precats, very common where the screen from one or both of them get into the main cat and block it.
Glad you found it.

Actually....he kinda did it to himself.

Header, RB catback, and a high flow cat.....with the air pump running directly to the cat to pass emissions....and never took it off.

He cooked the cat hardcore.

Yep, that would do it.
No problem with running the air pump, just need to run the air through some sort of ACV/Diverter valve. We used the Ford diverter, cheap and works. Should maintain a check valve also.