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2003 Forester X 5 MT
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Exhaust leaks are known to cause P0420 codes.

OEM Cats are $1200

You can google the Spark Plug Anti Fouler O2 Sensor for one option. It puts the rear O2 sensor further out of the exhaust stream that makes it think the Cat is working.

One way to trouble shoot is to take the temp of the forward cat and the rear cat after driving the vehicle (with it running). Use an Infrared non touch thermometer (Harbor Freight/Princess Auto). There should be at least a 100 Deg C difference if the cats are working
 

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2003 Forester
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Thanks, I ordered an infra red thermometer to learn more about how this works. Will double check exhaust leak. As far as price goes, any idea on why some cats advertised between $200 to $500?
 

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2007 Forester Sports XT 4EAT
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As far as price goes, any idea on why some cats advertised between $200 to $500?
My guess, aftermarket CAT... not approved for California or New York emissions.

Take a look at my search results on RockAuto.com :wink:

Bobby...
 

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2003 Forester
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Bobby; I am trying to reach the 21st century and am logging time on past postings on this subject. Today, after 161 miles, the CEL reappeared (finally, highway drive) (I/M OK) pending 420 cod went to simple 420 (hah). On the OBD scanner: the 02 sensor output voltage (B1-S2) indicates an ocillating : .6 to.85, the O2 sensor output voltage (B1-S1) indicates a 'steady' 2.0 to 2.2. Lambda B1-S1 .978 to 1.1. ( Back to studying Greek) . Do you know where to go to find out if these are 'normal' or abnormal' values. If I can find an exhaust gas tester, I'd like to see what it indicates, etc.

I am loathe to do anything more at this point (except to monitor things) because the car starts, idles OK, accelerates OK, runs OK and on this trip got 28 mpg. Tony
 

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2007 Forester Sports XT 4EAT
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Do you know where to go to find out if these are 'normal' or abnormal' values.
Peruse this thread, I believe some members have dug into this.

I am loathe to do anything more at this point (except to monitor things) because the car starts, idles OK, accelerates OK, runs OK and on this trip got 28 mpg.
Do the $5 fix & call it good.

We've had the $5 fix installed on my son's '03 X since the AVO turbo kit was installed. :wink:

Bobby...

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

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I've got a P0420 on my 2010 X which has 175k miles on it. Already replaced the downstream O2 sensor and it comes back just as others have experienced. I'm also seeing terrible gas mileage, around 15 constantly. My issue here is that I'm in California and I'm not trying to shell out a grand for a cat.

Everything other than MPG seems fine. No weird idle, runs fine, no choking or struggling.

Should I replace the upstream sensor, seafoam + italian tune up, clean the MAF, what direction should I go?

Thanks!
 

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2007 Forester Sports XT 4EAT
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Sounds like you're running too rich! If this has been going on a long time, the CAT(s) may have been damaged. :confused:

Assuming there are no other issues... you've cleaned the MAF sensor, checked for loose or cracked vacuum hoses & your Subaru recommended maintenance is up to date, it could be time to replace the upstream, front, Air/Fuel sensor.

One thing to remember, the Subaru engine runs better with Denso O2 sensors, which the factory installs. :wink:

Bobby...

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

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2001 Forester S
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A year or so ago, my 2001 Forester check engine light came on. Drove to a trusted Subaru mechanic in Carson City, NV and he found that the Cat is bad. the mileage then was around 99k. Have driven around 5k more and my gas mileage is shot. Just started happening....3/4 of a tank down and 146 miles. Is it the Cat or something else?? A bit worried here and mechanic has ordered a new Cat with a 5 year warranty. Any input appreciated!! Thanks!
 

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2007 Forester Sports XT 4EAT
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@Shirley Cap I see this is your first forum post, so welcome to the forum from Oregon! :biggrin:

Okay, your mechanic has ordered a new CAT.

I have some questions: :confused:

[1] If your CEL is lit, what CEL codes do you have?
[2] Has the front O2 sensor been replaced?
[3] Are the head gasket leaking?
[4] Is your Subaru recommended routine maintenance up to date?

Bobby...

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

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I haven't cleaned the MAF yet, but I did go over the engine bay and didn't see any cracked/missing hoses.

I installed a Bosch sensor in the rear, but I'll get a Denso for the front if I change it. The car has 176k on it, so it wouldn't surprise me if the cat is shot.

Maintenance wise, I bought it with 26 service records, so I'm pretty certain it's up to date :)
 

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2007 Forester Sports XT 4EAT
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The MAF sensor is very easy to clean.
You can use a MAF cleaner (CRC makes some) or isopropyl alcohol (rubbing alcohol). If you end up using a non-aerosol liquid, just pour some into a clean zip-lock plastic bag. Seal the sensor in and shake the bag by gripping the sensor housing. You'll probably be surprised at the gunk that ends up in the bottom of the bag! If it's aerosol, just spray (gently!) inside and out. Make sure you don't poke anything inside, or you could damage the sensor!
Questions: :confused:

[1] The P0420 CEL code is the only codes you have?
[2] How many miles on the spark plugs?

Bobby...

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

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I'll try to clean the MAF and either run some seafoam of cataclean this weekend.

P0420 is the only code, no rough idle or running, plugs have about 300 miles on them, when I bought the car I did plugs, wires, air filter, oil filter, diff fluid and... I think that was it.
 

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2011 CDN Convenience Pkg 4AT
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Mine went with only 58,000kms on it. The mechanic said that 95% of the time it is the catalytic converter. They do a test where they plug their tablet into the OBD-II port and drive it at a consistent speed over 80KMH and the readings between the two sensors has to be in a certain range if not...the converter is seen to have failed. Fortunately, it was covered under the emissions warranty.
 

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2014 Forester
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I have a 2014 Forester currently with 110000 miles. I never saw a Check Engine until about 100000 miles and the code was P0420. This was the first time I ever heard of an issue with an inefficient catalytic converter. It surprised me since I've had several cars with over 200000 miles and I never had to think about issues with the catalytic converter. Over the next few 1000 miles I did the following:

1) Replaced the downstream O2 sensor - NTK 24561 Summit Racing $65. Did not change anything.
2) Brought the car in for diagnosis from my favorite Subaru specialist - not the dealer. Diagnosis replace the converter - about $1250 total cost.
3) Contacted Subaru America who told me to take it to the dealer for diagnosis, with same result. Recommended replacing the converter - about $1800. Subaru America took about a week to decide that due to the mileage exceeding 100K they would not do anything. I believe they would have covered it under an extended warranty if the mileage was under 100K.
4) I decided to live with it. I performed the "$5" fix, using a spark plug non-fouler, which pulls the sensor out of the converter chamber about 1.25". I have not had a Check Engine since performing this modification.

As an aside, Subaru America did cover an engine replacement, due to excessive oil consumption, at mileage 75K.

ColoradoZ28
 

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I've never replaced a cat on any vehicle, do you really think this is the problem in your forester? Ferret had a lot of good info to back this up, but I never had personal experiences with this... I'll follow this since I'm curious
 

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2006 Forester 2.5X
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I have taken the time to read through this thread. I was going to post a new thread but decided this one covered just about everything concerning the P0420 issue except one thing I am seeing on my 06 Forester. Mine was throwing the P0420 code and I decided to change both O2 sensors at 206,000 miles since they were original. The problem went away for about 6K miles and then it started again at 212,000 miles. Maintenance is up to date, no coolant loss, 3/4 quart oil consumption in 5000 miles, no leaks in the exhaust, well maintained, 26-27 MPG. Here is my catch; my CEL only pops when my cruise control is on. If I drive the car 10 miles and then set the cruise, no problem. If I set the cruise immediately when I get on the highway (I live right on the highway), after about 7-8 miles the light comes on (usually). As you all know the cruise control is disabled with an active CEL so 3 runs after the light comes on, it goes back out on its own. I know the easy answer is to just drive it 10 miles before turning the cruise on but I am the type that wants to know why this is happening.
 

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2007 Forester Sports XT 4EAT
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@seanlivolsi did you replace both O2 sensors with Denso O2 sensors? :confused:

My first thought, the engine could be running rich... perhaps it's taking too long to get up to the normal operating temperature. Rich = more O2 in the exhaust flow = P0420 CEL code!

Also...
Have you checked for exhaust leaks near the O2 sensors?

Bobby...

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

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@2.5x_sleeper, yes both sensors were Denso sensors. I have checked for exhaust leaks, had the cone gasket and bolt/spring set replaced as a precaution so I am pretty sure I am leak free. I should note some additional details. When I leave my house I get right onto a highway. 5 miles later I drive up a 3 mile long mountain. If I leave and set my cruise control at 60 MPH, almost predictably the CEL comes on as I crest the top of the mountain (and of course my cruise is disabled). If I do the same thing and just wait until I am over the mountain to set the cruise, the light has been staying off (so far about 800 miles). From what I have read the ECU runs different checks at different points in the drive cycle. The P0420 check is done once per drive cycle, after the engine reaches normal operating temperature and once the vehicle speed is steady above 42 MPH. A lot of people say this is somewhere between 8-10 minutes of operation. Coincidentally it takes me about that long to get where I do when the light comes on and it always seems to be in the same 1/4 mile stretch of highway. The only difference is with the cruise set it will come on and it has not come on since I started waiting to set the cruise. I will say that since I replaced my O2 sensors, I gained mileage (was 25-26 and now 26.5-27.5) so I am not seeing a rich condition at the gas pump. I have monitored operation of the O2 sensors while it is driven and they both seem to be doing the right thing at the right time. The rear sensor runs between .7 and .8 going steady down the road which I thought could be high but I have been told that is a good number for highway speeds. The front sensor shows 2.7 - 2.8 at the same time. If I lift off of the gas the rear will drop towards 0 and the front sensor will go up to 5+ so they seem to indicate properly. The only variable I have seen so far is the cruise control. Its weird and may not be a factor at all. I even verified that the heater circuit in the rear sensor still has continuity and it does. I had thought about the "mechanical" fix because all other indications are good but like others have said, that can be masking a problem. I wouldn't be upset to replace the Cat at 213,000 miles but if it was really bad I would think I would be getting this code all of the time and I am not. It is possible that the fuel management under cruise control conditions is different than manual and at that point in my drive may be indicating a rich condition when the ECU runs the P0420 test. I don't know enough about it to know that. My other Subaru was a good example of a bad Cat; popped a P0420 code every 150 miles or so no matter what we did (fixed leaks, changed sensors, ran sea foam and other additives, etc). Replaced the Cat on that one and the code has never returned.
 

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@seanlivolsi forgive me if this has already been covered... check your MAF and MAP sensor readings. I don't know proper values off the top of my head but I can tell you my Subie mechanic neighbor says he sees these two sensors causing a P0420 very often. For us it caused a slightly rough idle, fuel odor, bogging, running rich, etc. I covered it in another MAF/MAP thread.
 
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