I'm getting a P0420 on a 2009 Legacy 2.5L (no turbo).  This came after an engine rebuild required after a ringland failure of #4.  Also, after the engine rebuild, the lower cat was found to be plugged with what I assume are parts of the manifold cat, so I suspect the manifold cat was damaged when the car was run with the dead cylinder sending unburned fuel to the cat.  Changing the lower/back cat restored power and the car runs fine except for the P0420 code.

Scouring this forum,  I found many references to checking fuel trims before condemning a cat for a P0420.  According to a cheapo scanner, the long term fuel trim is -7.7%.  Is this within range for normal fuel trim?

Thanks for any input.

John

Pretty normal. Depends on when you took the reading. A high reading MAF at idle is indicative of it being dirty. Negative idle trims and positive cruise trims are typical of dirty MAF and/or dirty air filter. 

GD