P0420 does NOT 'automatically' point to a bad cat conv. Though, a lot of people have probably been sold cats.

 

read here first before swapping any cats based solely on a P0420; http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diagnosis.html

I told him to actually look at the substrate material and see if it was damaged. If it's not damaged, then why would he replace it? Anyways, a vacuum gauge can help diagnose too.

I read once of a cat test based on pushing propane through the cat but, it might be some expensive equipment so, smaller shops might not have it. An exhaust leak can also cause the system to throw a P0420

Well, I have everything torn apart, heads come off tomorrow and head to the shop on Friday.  Hopefully they don't find anything wrong and it's quick.  Everything looks good so far, but Ic an't see real well with the heads still in the car.

 

Is there anything I can do to help prevent this from happening again?  Is there a better gasket than Fel-Pro MLS?  I have had the studs suggested a couple times, I don't want to have to pull the engine in order to put them in and do this repair down the road.  Also, do i need to replace the intake, exhaust, and throttle body gaskets again?  I did them last time (about 15k ago)

All back together with beautifully machined heads, clean block, new fluids, and all new gaskets.  Ran like a champ last night, no more white smoke, smooth idle, and cranked right up this morning!  Thanks everyone foy your suggestions, knowledge, and patience!  The gaskets were definitely bad, during the tear down and reassembly process, I noticed coolant mixed in with oil in the PCV lines and the gaskets were bubbled.  i'm not sure what happened, but something went haywire!  The top coolant line no longer remains under pressure as well.