I have an 08 f150. 290k 5.4 rebuilt 6 months ago at 280k
heard rattling first about 2 wks ago, thought it was loose heat shield. No performance issues. Yesterday out of nowhere, shaking and rattling, smelling fuel, exhaust sound changed. Got out and immediately smelled sulphur. Let truck cool, started back up to see if cat was glowing. No glow but drivers side definitely hotter than passenger.
i know its illegal. Dont need reminding.
So...
How do i gut it without removing? Will replace later after i find origin of fault. How do i determine the cause? I know i had a vacuum leak before engine rebuild. No other issues.

 

Gutting it is going to cause issues with fuel trims and likely keep your check engine light on. If you don’t want to remove it, the best way is gonna be to cut a hole in it then weld back. My recommendation is to replace it.

 

Not disagreeing with @nrivera04 but "No performance issues, no DTC or Code (mentioned at least), smelled gas, exhaust sound change, etc etc", and you have condemned a CAT. I would lean toward some more checking before slashing my wrist (so to speak).

 

I’d fix those misfires and get another cat converter.

I don’t believe low grade misfires trip the CEL. So, you may still have something show up with the right scanner.

 

Fix the cause first, not one of the possible symptoms.
If its shaking due to misfires, fix that first, then re-evaluate.

 

Are you sure there are no DTC codes? Any pending?