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06 Forester, 02 Tacoma
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I have an 06 5 speed with 170 on the clock. Had it a few months, passed inspection a few weeks ago the P0420 started. Scanner shows me that both O2 sensors are, from idle, responding to pretty much instantly to the gas pedal. The seem to be tracking pretty much perfectly, that is spike up and down in sync with each other and the pedal.

This is a Federal standards car.

Is this saying the Cat is bad? If so, which one? Any comments on the Bosal line?

Am I reading the FSM correctly? I have not checked the physical layout on the car, but it seem to be saying the front and rear O2 sensors are on the front exhaust pipe, before and after the front CAT, but BEFORE the rear CAT. Is that correct:?

I checked some parts illustrations and yes, the sensors bracket the front CAT. I guess the rear is just there for "clean up".

So, If I have the two sensor "tracking" and no exhaust leaks up front, I can presume the CAT is done, or needs a good cleaning? Davico advises using something called Sledgehammer to "decarbonize" before changing the CAT.

I now recall the previous owner telling me a bout changing a coil pack. From the way she said it I infer it had run poorly for some time. That may have carboned things up quite a bit?
 

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2007 Forester Sports XT 4EAT
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@joea99 the rear CAT is not monitored... which I thought was odd, figured it was added for States that have stricter emissions & uses a tailpipe sniffer as part of the emission testing.

On the diagram below, [6] is the front O2 sensor & [3] is the rear O2 sensor.

I always thought moving the rear O2 sensor after the rear CAT would prevent the likelihood of getting a P0402 CEL code.

Bobby...
528416

['07 FSXT Member Journal] ['03 X Member Journal]
 

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2006 Forester X 2.5
160k miles

This one has my stumped and I can't seem to find a answer. Got this code and started digging with my reader and got some confusing readings.

2.799v on upstream O2 sensor
Downstream O2 had large voltage changes (Which to my knowledge indicates a bad cat)
Used thermometer on cat and showed 300 inlet and 500 outlet (something close to that it looked good to me)
replaced front O2, same voltage reading
tested old o2 with torch got typical .000 to .900v
now that upstream O2 still shows that higher voltage when it is hot, cold, off and even unplugged which I am not tracking why its reading that.

P0420 is the only code I had (I reset it)
Car drives great, no performance issues or funny smells just that strange voltage reading I am used to quick movements from low to high on that front sensor
 

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2012 Forester 4 speed auto
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1,108 Posts
@xA2D9 That's because it isn't a normal O2 sensor. It is a Air Fuel ratio sensor and the voltage can be 2.4 to 2.8.V The down stream O2 sensor is a normal O2 sensor and should vary between 0 and 1 Volt. Check your fuel trims next.
 

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06 Forester, 02 Tacoma
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145 Posts
@joea99 the rear CAT is not monitored... which I thought was odd, figured it was added for States that have stricter emissions & uses a tailpipe sniffer as part of the emission testing.

On the diagram below, [6] is the front O2 sensor & [3] is the rear O2 sensor.

I always thought moving the rear O2 sensor after the rear CAT would prevent the likelihood of getting a P0402 CEL code.

Bobby...
View attachment 528416
['07 FSXT Member Journal] ['03 X Member Journal]
I only work on my own vehicles but have replaced a few O2 sensors. Everything I have worked on at least 2000 and newer, has at least two CATs. Some transverse V engines have one for each bank and one "rear". On Toyota, the rear O2 is after the rear most CAT

Does seem like Subaru is making it hard on themselves doing it this way.
 

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2005 Forester X and XT Manual and Auto
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1,046 Posts
@NinoS I don't believe those spark plug non-foulers are designed for constant exposure to the elements... like salty winter road spray, so that might have been the reason yours rusted & seized?

No winter road salting here where I live, so the one we installed in my son's '03 X is still rust free.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
I think you are spot on with this. We dont get a ton of snow but we do get one or 2 events a year, plus living near the ocean too doesn't help. We are year round humid as well. Certainly closer to rust belt conditions than not.

On the generic cat, i checked it out when replacing the starter last week and it looked surprisingly good...pretty much like new...but again has not been long. Will serve its purpose for us at least.
 

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06 Forester, 02 Tacoma
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@joea99 the earlier XT had 3 CATs! What surprised me is the "unmonitored" rear CAT. Even more curious, I have seen some aftermarket replacement that do have a bung on the rear CAT.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
Wonder how moving the sensor would work? Stainless Bungs are available, have one on hand, so I could try pretty easily, if the cable reaches.

I'm running some cleaner in the gas right now to see if it takes longer to set, or if the O2 voltages look different after. A few miles after the cleaner went in, I could swear it is running smoother and revving better. But I am sorta gullible . . .
 

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I have a 05 5 speed x that is having a p0420 issue. 206k miles and I just replaced head gaskets, coil pack, ignition wires and plugs. Previously, it had been run by the old owner with a bad coil pack and failing head gaskets for some time and was misfiring off and on. So the cat almost certainly had some coolant leaking into it. (Old owner replaced original Cat with aftermarket at around 187k)
Right before I replaced the coil pack. It had misfired excessively and barely made it home after a short drive and the cat seemed to be smoking a bit from near the downstream 02 sensor. I had a P0420 and P2906
Since then, I swapped both O2 sensors with some older ones I had that I believe are still good.
Then only the P0420 code showed up. Next I tried the $5 fix with two defoulers. Now only P2906 code shows.
Mpg is about 4 miles per gallon worse than normal, but otherwise the car runs well. Should my next step be to replace the upstream sensor with an OEM? Or just replace the cat with a Eastern Cat?

Thanks for the help.
 

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2007 Forester Sports XT 4EAT
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@mvnichol welcome to the forum from Oregon!

Take a look at this post in the $5 fix thread.

ALso...
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Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 

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Forester 2001 L - Stock
101k Miles
P0420 - Catalytic Converter Efficiency Below Threshold

The forester has been throwing this code for a few months now, typically shows up on long highway runs (but to be fair, that is the vast majority of my driving). I bought a scanner from scantool.net, which I am quite impressed with for $60 new. This is the only code that's been thrown, so I am assuming it's the cat, I just want to gather what insight I can before going through with the repair.

I have seen some posts saying it could be the rear O2 sensor failing. How likely is this, given it has not thrown its own code? I have called around the area, but nobody here can do an exhaust check to find out for sure. I've also heard that an emissions check might not give any useful information, since the efficiency code is thrown well before the problem gets out of hand.

Assuming it is the cat, is there a good way to determine if it is just the front or rear unit, and replace individually? Also, can anyone offer any insight on the best replacement unit? I saw TexasForester's excellent writeup on the Eastern #40237 and his issues with that unit. Does anyone have any experience with other possibilities?

Any help is appreciated... Thanks!
Had exactly the same problem, bought an extender for less than $10 and cured the problem straight away.
 

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2001 Forester S
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4 Posts
I'd like to put my 0.02 in on this situation as I recently encountered this problem. You can skip down to the last paragraph if you wanna bypass all the background stuff and read my eventual solution.

Background (because i need to contribute meaningful posts per forum rules)
I have an 2001 Forester S (USDM), that I bought with a blown head gasket last October. After after a few weekends, i was able to get the foz back together and take it to the smog check place. Having never had smog inspection issues with an OBDII car, I was instructed, after failing the test, to drive the car for 100 miles and return to get retested.

450 miles and two failed smog checks later, I was on my way home from work when the readiness monitors finally cleared and the P0420 appeared. "Fine," I said. Then I spent the next 4 days trying different catalytic converter cleaners and additives.

3 months, 1 timing belt, and 8 bent valves later, my opportunity to investigate the cause of the P0420 returned at last. First I replaced the O2 sensor, and drove the car for 2 days, while the light remained. I reset the codes and went through the drive cycles again (just in case the o2 sensor was the problem). 3 days later P0420 returned. Next, I tried filling the gas tank with 91 octane, along with the spark plug defouler for the rear O2 sensor. I drove the car for a day, while the P0420 made itself comfortable on gauge cluster. I cleared the trouble codes again and spent another 2 days going through the drive cycles upon which the P0420 came back. This whole time I was driving with expired tags and an out-of-state plate, because I needed the smog cert to finalize the title transfer, so I was really anxious and stressed every time I had to test drive the car, or go through the different drive cycles.
I was comparing the O2 sensor and A/F ratio sensor graphs during this whole time, and the graphs indicated that the catalytic converter was performing typically. I had a spare wideband that fit the Subaru harness, so I figured i'd swap that sensor and see how that went. 3 days and 3 drive cycles later, the P0420 - like an annoying roommate that sits in front of your computer, and goes through your AIM message windows without asking, popped back in. After reading some opinions on the O2 sensors, I opted to buy a genuine subaru wideband, just in case.
4 days, 1 subaru wideband sensor, and 3 drive cycles later, I have clearing subaru readiness monitors down to a T. And the wide-band that came with my AEM failsafe gauge is looks to be the same model as the subaru wideband, except the resistors are different. But, the P0420 code was still there. I wanted to exhaust all of my options before throwing down $500 on a cat for a $300 car.

The Solution
After reading that Cataclean was comprised of lacquer thinner, and that the exhaust stream temperatures are the parameters that need to be manipulated, I opted to go for the Scotty Kilmer Lacquer thinner route. Before heading off the buy the thinner, I started the morning with an Italian tune-up and drove around for a half hour, while my wife and daughter ate the breakfast I made for them (so I could go play with my car). At this point, I had about 6 gal. of 91 octane in the tank, so drove to the gas station and added about 4 gal of 100 octane, to have enough fuel to dillute the lacquer thinner. I put a gallon of thinner in my 10 gal mixture of 91 and 100 octane gas, and drove around for about 30 minutes - but the P0420 still present. Feeling defeated, invited my wife and daughter to join me in eating away my sorrows at an all-you-can-eat sushi restaurant. As I was going through the stages of grief, I figured that I would reset the DTCs (just-in case) and drive to a restaurant that was about an hour away from my house, just to have enough opportunities to clear the readiness monitors. On the way to the restaurant, my wife was operating the scan tool, while I went through the drive cycles; and after about 25 minutes, we were notified that the P0420 was pending. Struggling to hold back the f-bomb so as to avoid tainting my toddler's ears, I accepted the truth that the reader was telling me and the girls and I had our fill of tuna - for 6 people.
On the way home, my curiosity started nagging and I asked my wife to run the diagnostics. Lo, and behold, the readiness monitors were cleared, and there were no pending codes. Since my daughter was in a sushi coma and my wife was about ready to join her, I dropped them off at the house and drove to the smog check station and got passing certificate. It's been 3 weeks and still no P0420.

My guess is that the catalytic was in good condition until my foz's previous owner blew the head gasket and drive it around like that for some time. I imagine the combustion of the coolant/fuel mixture was fouling the surfaces of the converter, which is probably why the lacquer thinner helped with my case.
 

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2005 Forester XT
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CLEAN Your CAT - Fix stubborn P0420
Here's what I did: On 2005 Forester XT with P0420,
1. Removed downpipe/catalytic converters.
2. Separated into upper Cat and lower Cat.
3. In a 5-gallon bucket, mixed 4 gallons water, 10 oz. Citric acid and 7 oz Oxalic acid. (Purchased on Amazon - $20)
4. Soaked main Cat in acid overnght, rinsed completely with water.
5. Soaked upper Cat about 8 hours, then rinsed completely with water.
6. Reinstalled Cats, ran engine until hot, then cleared P0420 and it never came back. DONE.

P.S. For those of you that may be concerned about handling ACID, don't worry, just WEAR RUBBER GLOVES.
The oxalic acid is what's in "Bar Keepers Friend" cleaning powder that you buy at the grocery store, and
citric acid is commonly used in food to give it a "tangy" flavor.. Relax - it's not dangerous at all.
CAT Clean_01.jpg CAT Clean_Citric Acid.jpg CAT Clean_Oxalic Acid.jpg CAT Clean_Oxalic Acid_02.jpg CAT Clean_RINSE AFTER ACID Each Cat.jpg CAT Clean_SOAK IN ACID 12 Hours Each Cat.jpg
 

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I've tried reading through some of this thread and couldn't find this covered, but my apologies if it is.

I've got an '04 XT. Catless up pipe (from 05 STI) with resistor mod and STI TMIC installed recently. Was told I didnt need a tune. The front o2 sensor was also just replaced. A few hundred miles after these mods I got the P0420 code. The interwebs seems to have 2 categories of opinions on this one:

1) the catless up pipe only can't cause the P0420 code (has to be the downpipe cat(s))
-->if it's not causing the P0420, the fixes put forward seem to be
a) replace front o2 sensor (already done)
b) replace rear o2 sensor
c) the cat is bad so it needs to be replaced (or remove and soak and clean it).

2) the catless up pipe only can cause the P0420 code because there could be a small difference in the readings between the front and rear o2 sensor caused by removal of the up pipe cat.
-->If it is causing the P0420 code, then it appears the only way to "fix" this is to reprogram the ECU and turn off this code

Has anyone had P0420 come up in this type of scenario (catless up pipe only). If it's #1 then it sounds like this could be a driveability (or at least MPG) issue and I need to get it taken care of quickly (should I start with replacing rear o2?). If It's #2, it sounds like I shouldn't really worry about it except when I need to pass emissions every couple of years.

Thoughts/guidance?
 

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2007 Forester Sports XT 4EAT
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@sonoran_fozzyroo the P0420 CEL code originates from the rear O2 sensor... basically the only purpose of that rear O2 sensor in your '04.

Have you looked at this thread?
Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 

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@2.5x_sleeper I did see that thread, but I don't want to do the "$5 fix". Here's what I gather from your posts

1) "The P0420 comes from the rear O2 sensor detecting too much O2 in the exhaust flow. That can be from the engine running too rich, overwhelming the CAT or from an exhaust leak... it only takes a pin hole."

2) In an '04 XT, the rear sensor is not used for fuel mapping and is just to monitor operation of the CAT.

From this, I'm still not clear if the catless up pipe can cause this, or if I should be concerned that either my engine's running too rich or the cat is bad in the downpipe. Thoughts?
 

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2000 4Runner
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Blow back air gets the rest of the undercarriage really dirty/nasty. As it was the case of my '01 L before the H/Gs were replaced and which is still giving me the P0420 code.. This is how dirty my transmission oil pan was before I overhauled the engine.. more than likely there's still dirt/oil around the 2nd CAT. I will try both cleaning the CAT and the $5 fix method see how it goes.. I know there's nothing wrong with my engine, but that ENGINE FAILURE light is just really annoying.

531763
 

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2007 Forester Sports XT 4EAT
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@2.5x_sleeper I did see that thread, but I don't want to do the "$5 fix". Here's what I gather from your posts

1) "The P0420 comes from the rear O2 sensor detecting too much O2 in the exhaust flow. That can be from the engine running too rich, overwhelming the CAT or from an exhaust leak... it only takes a pin hole."

2) In an '04 XT, the rear sensor is not used for fuel mapping and is just to monitor operation of the CAT.

From this, I'm still not clear if the catless up pipe can cause this, or if I should be concerned that either my engine's running too rich or the cat is bad in the downpipe. Thoughts?
If you didn't get a tune, after you installed the CATless uppipe, that less restricted exhaust flow could be throwing off the ECM A/F - Air/Fuel setting. If so, that could be causing your engine to run too rich. The rear O2 sensor will detect too much O2 in the exhaust flow, giving you that P0420 CEL code.

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 
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