JohnCTT

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

I had one in my daughter in law's 09 2.5 at less than 100K miles. But, that car head a dead number four. It was literally running on three cylinders. John

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

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? .

I'll have my son check the short term trim when it's fully warm. Should that be taken at a specific rpm in neutral, or should it be under load? BTW, I just checked my Mazda with my scanner, and the STFT changes constantly, although my cheap scanner updates once every 5 seconds. There are no codes stored and no CEL on. Thanks for the help. John

Do you mean don't hone at all (leave as is) or do a full cut? Anyway, I didn't completely understand the AF description, but I had my son check the fuel trim with his scanner, and he said it had a Long Term Fuel Trim 1 of -7.7% and a Long Term Fuel Trim 3 of -100%. Since this has a single cat after the collector, this is a one bank system even thought it's a boxer, correct? I assume the Trim 3 is meaningless. Does this lean towards a bad cat? Also, you used the term AFR sensor. Does the Subaru use a full range sensor or a standard binary O2 sensor? John

Maybe I'm looking at this wrong, but it seems the upstream O2 is in the collector before the cat, but the "downstream" O2 is actually in the **middle** of the cat. {???} John

Forgive my Subaru illiteracy... is the 09 2.5 Legacy one of the newer ones that use the downstream O2 for AF ratio? John

That's what my intuition tells me, but if you Google P0420 codes, every return will mention intake leaks! Doesn't make sense to me either. Anyway, the Subie exhaust is a one piece all welded affair, is it not? I'm guessing a bad cat, but the exhaust gaskets from the heads on out can only cause a lean code, not an efficiency code? Thanks for help guys. John

Well, I got a P0420, but I'm still using the original cats. Here's what I don't get; in researching P0420 codes, many people talk about intake leaks and such. In my experience, and air leak will cause a lean code. I don't understand how an air leak can cause a normal response for the first O2, but cause an efficiency code. Am I missing something here? Because the car had a misfire from the blown piston, I'm assuming the cat on that bank was damaged and that's what's causing the P0420 efficiency code. I don't mind replacing the exhaust manifold/cats but don't want to if that's not the problem. BTW, I had to reregister because every time I try to log in, it always says wrong password and locks me out. So then I have to wait for the time out, and then go through the process of changing passwords. Unfortunately, it hasn't taken the same password twice.. Hopefully my new account will work. Thanks. John