Hey guys, I installed a brand new Subaru short block in my daughter in law's 2009 Legacy 2.5 non-turbo about 6 months ago because it had a broken piston:

 

It's running normally (thank God)  but keeps setting a P0420 cat efficiency code, so I need to replace this.  I already have over 5K in this car and would like to minimize the cost of replacing the cat if it's reasonable to do so.  So I have two questions: are the aftermarket cats safe bets to work properly for a few years without setting codes or is the OEM cat/manifold the only safe option?  If the OEM is the only way to go, is there an on-line site that has discounted prices?

Near as I can tell, the OEM is going to cost me $900 but there are aftermarket solutions for less than a fourth of that.

Thanks for any insight.

John

EDIT:  On other cars, I've used O2 spacers to eliminate the efficiency code , but the downstream O2 sensor seems to be in the *middle* of the cat on this Subaru.  Am I seeing that correctly?  Can a spacer be used successfully on these cars?

 

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Edited September 4, 20196 yr by JohnCTT

First make absolultely certain the converters are bad, there's no exhaust or vacuum leaks from the recent engine work - it's not uncommon to have one leaky hose or exhaust flange gasket, etc. 

Generally speaking spacers aren't as effective in 2005+ models, but im' not sure on specifics.

I believe the later vehicles use the rear O2 sensor for fuel trim so the spacer can set other codes.  There is a great P0420 code thread over at Outback forum

https://www.subaruoutback.org/threads/p0420-diagnosis.49537/#post-474237

It's long but the first few pages have the good info.  May not be a cat but if it is OEM is the way to go.  Others don't last as long.  Some have P0420 code return in a few months.  Main issue is to figure out why the code is set before replacing the cat.

12 hours ago, Mike104 said:

I believe the later vehicles use the rear O2 sensor for fuel trim so the spacer can set other codes.  There is a great P0420 code thread over at Outback forum

https://www.subaruoutback.org/threads/p0420-diagnosis.49537/#post-474237

It's long but the first few pages have the good info.  May not be a cat but if it is OEM is the way to go.  Others don't last as long.  Some have P0420 code return in a few months.  Main issue is to figure out why the code is set before replacing the cat.

Thanks Mike, I'll go through the list.

This car though had a blown piston and bad skip, so it seems clear the cats were stuffed with raw fuel from the dead bore.   My daughter in law was out of town and drove it with the broken piston until it quit.  In addition, the second cat was replaced right after I rebuilt the engine because it was plugged with debris from what I assume is the manifold cat innards, so I'm pretty sure a fair percentage of the manifold cat substrate broke up.

Still, going over that list, I still can't see why an exhaust or intake leak or injector issue would set a cat efficiency code but not a lean/rich condition.  Stranger things happen I suppose.

John