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Hi All,

My P0420 code started this past February and it has been driving me crazy. I started with replacing the pcv, code still there, then I used the Seafoam method. That cleared the code for a few weeks just enough to help me pass my inspection in August. After that the code has been coming on persistently. I don't have drive-ability issue, the car runs like a champ, no lost of gas mileage whatsoever when the code is on.

So this past 3 weeks I have been dedicated my time to chasing this problem. I cleaned the EGR, my throttle body, flushed the radiator fluid, check the thermostat..no go. Replaced the spark plugs, new spark plugs wire. Tried the anti-fouler, first using 1 then using 2, that made it worst, throw me 3 more codes P2096 and P2098, and P0137. So I took the anti fouler off. Replaced the front sensor with Subaru/Denso sensor..no go, replaced the rear sensors with Subaru/Denso sensors, no go. I called the dealer, they wanted $2600 to replace the cat, not going to spend that much money on an 04 with 146K miles. So before I replace the cat converter which we all know is expensive even if I do it myself, I threw a hail mary and tried the diode hack. Viola!!!

It has been 3 days now with 6 drive cycles and no check engine light, all system back to ready when I checked it this morning with my code reader. Finger crossed. Here is the link to the hack

2000 Subaru Outback P0420 Electrical Hack Dan's Toolbox

Sorry for the long post and I hope this will help others that are having the same problem as me.
 

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This is the "fix" used when an engine is tuned for high boost levels and rich fuel mixtures with catalytic converters removed from the exhaust system to trick the computer into thinking the cat is there and functioning.

No offense, but all you accomplished was tricking the ECM. You still have a problem and at some point, it will rear its head.

Have you looked at http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diag.html ?
 

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This is the "fix" used when an engine is tuned for high boost levels and rich fuel mixtures with catalytic converters removed from the exhaust system to trick the computer into thinking the cat is there and functioning.

No offense, but all you accomplished was tricking the ECM. You still have a problem and at some point, it will rear its head.

Have you looked at http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diag.html ?
Hi cardoc,

no offense taken. i did read and watch your post before i started chasing this down, very helpful. i did most of the items you mentioned like tune up, checking my oil, replacing the radiator fluid, checking exhaust leak etc etc and none of these fix the code.i wish we have tech like you up here in NH. i did call the dealer to ask them for diagnostic but they said all they do it hook it up to a computer and watch the graph or some sort to determine but they are very adamant about replacing the cat.

i am hoping to find an indie subaru repair shop to get this check out and i am also hoping / crossing my finger to have save enough money in another month or two to trade it in and get the new Crosstrek.

i do appreciate your post on the P0420 diagnostic, which is very helpful
 

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Just a quick note on P0420, I had this code just before the P0032 (bad heater on forward O2 sensor) took over eliminating P0420.
 

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For some reason my OBS-t would not go into ready mode with an electricic CEL "fix".
So
I've used 2 spark plug non-foulers on the REAR o2 sensor (1 untouched screwed into the exhaust then 1 drilled out to allow the rear o2 sensor to thread into it (2 have to be used together to eliminate the cel)) for many years & get no CEL + all systems are "ready".

This has worked on EVERY car I've tried it on - Hondas, Toyotas, Various Subarus.
 

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Hey Toro168, thanks for the post. I am having the exact same issue on my wife's 08 subaru outback. I had the P0420, installed the anti-fouler, and then started getting the P2096 and the P0137 which is what brought me to your post. I am going to give this a shot, but have a few questions. Has your CEL stayed off since the fix? Also, I was thinking about just taking out the rear O2 sensor and installing the diode on its wire in the engine bay. Is that where you installed it, or did you go back inside the car and do it there? Thanks!
 

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Hey Toro168, thanks for the post. I am having the exact same issue on my wife's 08 subaru outback. I had the P0420, installed the anti-fouler, and then started getting the P2096 and the P0137 which is what brought me to your post. I am going to give this a shot, but have a few questions. Has your CEL stayed off since the fix? Also, I was thinking about just taking out the rear O2 sensor and installing the diode on its wire in the engine bay. Is that where you installed it, or did you go back inside the car and do it there? Thanks!
In your case, you can buy the diode and spend the time to splice it in, get the anti-fouler and work that in the exhaust, or you could check the O2 sensor fuse, SBF5 in the relay box, the O2 relay itself to make sure it is working and if these are good, just buy a new rear O2 sensor. Its not working.

There is not a quick fix for anything. The core of the issue needs to be located and repaired. In your case the computer isn't getting anything or is always getting a low reading from the rear O2 sensor.

Another thing that causes this if the O2 sensor is in fact good is a clogged CAT and no anti fowler or diode will correct it.
 

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Hey Toro168, thanks for the post. I am having the exact same issue on my wife's 08 subaru outback. I had the P0420, installed the anti-fouler, and then started getting the P2096 and the P0137 which is what brought me to your post. I am going to give this a shot, but have a few questions. Has your CEL stayed off since the fix? Also, I was thinking about just taking out the rear O2 sensor and installing the diode on its wire in the engine bay. Is that where you installed it, or did you go back inside the car and do it there? Thanks!
Hi

The CEL has been off since I did the fix. I did exactly the way you are going to do. I took the sensors off and splice it close to the connector in the engine bay. That way I was able to keep the splice from getting wet. One thing you want to pay attention is the which end of the diode you splice.

Good luck.
 

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Hey Toro168, thanks for the post. I am having the exact same issue on my wife's 08 subaru outback. I had the P0420, installed the anti-fouler, and then started getting the P2096 and the P0137 which is what brought me to your post. I am going to give this a shot, but have a few questions. Has your CEL stayed off since the fix? Also, I was thinking about just taking out the rear O2 sensor and installing the diode on its wire in the engine bay. Is that where you installed it, or did you go back inside the car and do it there? Thanks!
Sounds like a problem with the rear O2 sensor getting power which may be as simple as a dirty / loose connector but I found this:

code P2096- wtf - Page 2 - Subaru Forester Owners Forum

Look @ the last few posts as guy on this site has the exact same problem you have.
 

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Thanks for the replies. I am only getting the P2096 and the P0137 when I have the anti-foulers installed. When they are installed, I don't get the P0420 code. As soon as I take them out, I start getting the P2096 and P0137 codes. I will read up on the other posts, but toro168 seems to have had the exact problem as I am having, so I will probably give the diode a shot. I recently replaced the rear 02 sensor and experienced the same results. I am in Oregon and we don't have to go through inspections where I live, so I am not too concerned about that. Any ideas on how much an aftermarket CAT runs for a 2008 outback?
 

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08? See, knowing the year and models helps. Call your dealership and give them your VIN and check on the ECM update to correct a P0420 and a CAT recall. If you are under 80k, its covered under the emission warranty. Even if you are higher, you should be covered.
In his defense, he did say "I am having the exact same issue on my wife's 08 subaru outback." It was the post right before yours. ;)
 

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I tried going to the dealership first and they did install an update to the ECU. However, since the car has 86K on it, they said the cat was no longer under warranty. As another note, the CEL will typically only come on every a few hundred miles, so whatever fault it is sensing, it doesn't happen that often.
 

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I tend to speed read the post.

I'll look it up concerning the CAT. As for it being an intermittent code, thermostat, type of gas, PCV or faulty rear sensor. The CAT could be deteriorating slowly and it's unable to store oxygen to assist in re burning and converting the HCs.
 

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Mine finally popped up again last night after being out for almost a month since last clearing. I just pulled freeze frame data:

Fuel Sys 1 CL
Fuel Sys 2 CL
Calc Load (%) 22.4
ECT(ºF) 190
STFT B1 (%) -3.9
LTFT B1 (%) 9.4
STFT B2 (%) -3.9
LTFT B2 (%) 7.8
MAP(inHg) 18.3
Eng RPM 2105
Veh Speed (mph) 57

I should note: I was out of town over the weekend, and I got gas from a "Bob's Quick Mart" or something like that, but the light didn't pop up until I was nearing the end of that gas, a little before the low fuel light came on.

We don't have many choices around here for tier 1 gas. There's a Sunoco station about 5 miles away, but the rest around here is typically Sheetz (a huge monopoly of a station in my town), and they're known for using the cheapest gas they can buy.
 

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Thanks for the link to the article. It was an interesting read. I will replace the thermostat since it doesn't cost much to give it a shot. The only other item besides the cat on your list is gas type. We typically only put 87 in there, although I know it calls for higher octane. Do you think that is the probable cause? Also, do you think an 02 sensor stimulator would eliminate the CEL from coming on, although obviously it would not solve the issue?
 

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I talked to Subaru of America today, and they simply said that it is not under warranty anymore, even though I complained that it is a common problem, etc. The funny thing is that she stated they have an extended warranty of 150k for the cat in several states (California, New York and a few others) for the 2008 year, but since I live in Oregon and they didn't adopt the increased emission restrictions until 2009, I only get the 80k warranty.
 
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