[TurboGT]

Yes thats the symposer plug. You no longer have that system installed. As mentioned before, dielectric grease the plug and seal it up with tape or something to keep dirt out.

 

[TurboGT]

Can you take a picture of the cat, please? I'm wondering if the heat shields were rusty and fell off when the dealership did the engine replacement.

In that case, it would be as simple as the rack itself was going into an over-temperature mode.

Nice find, OP!