muffler noise or hesitation rattle sound
engine light on troubleshooting code when check does po401 damage engine
I think also there exaust leak in muffler cause of smell can it trigger po401
Start and run engine at idle air shold escape form filter in pic. Check resistance of piece with engine off it shold be 33-39 ohms at 68 degrees then check for continuity between each pole and body of valve if there is replace it.