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2009 Forester 2.5 Premium Crappy 4EAT
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That is a project for this weekend. Now that the car is inspected, I am going to remove the non-fouler, as I do not want to do any damage to the car from running leaner than normal. It is a 2009, NA, 4EAT. I just bought an infrared temp scanner.
 

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Our 2000 Forester started doing this two years ago. Our local independent Subaru shop said this is typical for Foresters and if there are no other issues, just delete the code and don't worry unless it stays on.. I bought an inexpensive code scanner. The Forester (at 161,000 miles) shows this code at intervals of 3 to 4 months. I just reset it with the code scanner. No other engine or performance issues. Sometimes it shows up if the gas tank is overfilled. Other times there is no apparent cause.
 

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

Assuming you're not having any other issues, like poor gas mileage or reduced engine performance, this thread might be of interest to you:
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Bobby...

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

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2015 Forester 2.0 XT
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Hey everyone, I have a '15 Forester XT with the dreaded P0420 code. Troubleshooting it is a little beyond my skill, but I did buy a code reader and took pictures of the data stream for both O2 sensors in hopes someone here might be able to guess what's up.

I bought the car just over a week ago, and the dealership filled it up with regular gas when they topped everything off. My hope is that the issue is simply the low-octane gas (especially since this is a turbo engine), however I can't be too sure. Yesterday I added some octane boost (the tank is still half full, so I haven't been able to fill up with premium gas yet) but only drove ~6 miles since then.

Symptoms:
  • P0420 code
  • CEL on, Hill Assist light on, cruise control disabled, S mode
  • poor gas mileage (avg. 20 on highway, 16 in city)
  • even at running temp the exhaust seems steamy (visible steam coming out of them), but I might just be overthinking it
  • exhaust pipes have some soot in them. They were like this when I bought the car.
Other than the above the car runs great. No stuttering, it accelerates smoothly, etc.

Images of the O2 sensor voltage reads below.
Idling, with 1 instance where I revved the engine just a bit:
IMG_6045_noted.jpg
After clearing the DTC:
IMG_6047.jpg
I have zero idea what a "normal" or "healthy" data stream would look like, so forgive me if nothing stands out. I'm hoping someone here can really just tell me if I should be alarmed by anything here, or if the data stream won't tell me anything and I need to keep troubleshooting.

Thank you!
 

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

If you bought the Forester a week ago, take it back to the seller & have them address this problem.

Did the P0420 CEL code return after you cleared?

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Bobby...

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

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2015 Forester 2.0 XT
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@2.5x_sleeper Thanks Bobby! Glad to be here!

The dealership is currently giving me the "well, you bought the car as-is, so there's not much we can do for you" runaround at the moment. I'm hoping to know definitively if they sold me a car with a known issue or not before doing anything else with them.

The code has returned after being cleared once so far. Since then, I added octane boost to the regular gas still in the tank and cleared the code again. So far so good, but I've only driven ~20-30 miles so I wouldn't expect the light to come back on yet even if it's still an issue. The good news: gas mileage seems to have gone up substantially (+3 mpg/city, +6 mpg highway) which makes me hopeful!

Next step is to fill the tank with premium gas and add Seafoam cleaner (just to be safe). If the CEL comes back on again, I imagine I'll have to take it into a mechanic to see if it's a bad cat.

Am I missing/not checking anything else?
 

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2007 Forester Sports XT 4EAT
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@brntn the P0420 CEL code comes from the rear O2 sensor that monitors the efficiency CAT. That CEL can occur if the engine is running overly rich, overwhelming the CAT.

• If your gas mileage hasn't changed... reduced, then it's possible the CAT is failing.
• If your gas mileage is poor, then the odds are good the engine is running too rich & you need to direct your troubleshooting in that direction.
• If the engine has been running overly rich for a long time, the CAT could be damaged?

Bobby...

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

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2001 Forester 4EAT
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Hi guys.. I too have the same code P0420 on my ‘01 Forester L, 120K miles.
Since the first O2 sensor is critical(air/fuel mix) as Bobby quoted before, is there a way to know which cat/O2 sensor(1st or 2nd) has the issue? Or is the P0420 code directly related only to the 2nd cat/O2 sensor? Don’t want to end up stranded on the side of the road if it’s an issue with the first cat.
Update: Didn’t read the last post before me where it is stated the code comes on from the rear cat/sensor.. that’s a relief.
 

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2007 Forester Sports XT 4EAT
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@SubiAficionado normally, a P0420 is not going to leave you stranded, unless there's some serious engine issues that caused this CEL code.

If you're not having performance issues... reduced gas mileage or lack of power, you might check out this thread:
As to which CAT is at fault. That gets complicated, as the placement of the rear O2 sensor varies. Take a look at the exhaust diagrams below.

Bobby...

The diagram below is from opposedforces.com.
[20] & [21] is the front O2 sensor. [16] is the rear O2 sensor, note that it can be placed in 2 locations!

My Subaru technical service document shows on the '99-'02, the rear O2 sensor is placed directly after the rear CAT, so it monitors both CAT converters. You'd have to do a visual inspection to see where the rear O2 sensor is placed on your '02.
527305

The diagram below is from the '99-'02 Subaru Technical service document.
[23] is the front O2 sensor & [24] is the rear O2 sensor.
527308

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2001 Forester 4EAT
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Yeah, I’m getting regular mileage on my ‘01 L, typical of an 18 year old car I guess you could say, about 19-20 mpg. After the rebuild I did on my engine from the infamous HG issue, it has been performance great, no issues at all. I’ll look into the $5 fix see if the check engine cel light goes away completely. I check it from time to time with my scanner and it’s always the same code. Thanks Bobby
 

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2007 Forester Sports XT 4EAT
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@SubiAficionado sounds like that $5 fix might be the way for you to go. My son's '03 X has had the $5 fix installed since the AVO Turbo kit was installed. It got rid of the occasional P0420 caused by the 5th injector (throttle body injector) kicking in under WOT!

Bobby...

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

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2007 Forester Sports XT 4EAT
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@Wagonation that is another option, but note what I posted on this post in the $5 fix thread.


Bobby...

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

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2005 Forester X and XT Manual and Auto
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803 Posts
For the SH and SG folks, i had to replace the cat on my 03x due to the anti fouler trick backfiring years laters (rusted off, then threads rusted into cat). Found this one for $120 on amazon and it fit without any major drama...

MOTOOS Catalytic Converter with Gaskets Front & Rear Fit For 1999-2006 Subaru Impreza Fit Forester Legacy Outback 2.5L H4 https://www.amazon.com/dp/B07JVRQ2JC/ref=cm_sw_r_cp_apa_i_4ZbcEbYPCT42Y
527909

More info to share.....On my SG XT, I was getting cat related codes due to the crap turbo it came with leaking oil like crazy into the exhaust. Once i took care of the turbo and cleaned or replaced the rear O2 sensor, problem solved.
 

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2007 Forester Sports XT 4EAT
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@NinoS that's a killer price for a CAT! There are some negative Amazon product reviews... rates 3.9 out of 5. I'll be looking forward to seeing how this holdup for you!

Curious... on the previous installation, did you coat the spark plug nonfouler with antiseize when it was installed?

Bobby...

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

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2005 Forester X and XT Manual and Auto
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@2.5x_sleeper Hi Bobby! I'm pretty sure I can say "yes I did" on the anti seize but it was so long ago when I installed it's possible i'm remembering incorrectly. When the problem happened, my initial reaction was the particular anti fouler i had used was just total junk, it rusted out so badly. Maybe back then i didnt bother with the antiseize? Its possible.

All i can say on the cat is so far so good (maybe 6 months). Its a good point though as I'm sure it won't last as long as oem. The plan for that car is to get it through my daughter's current senior year, then get her something else with maybe 150k less miles for college:LOL:, and take that car back and figure out what to do with it.
 

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2007 Forester Sports XT 4EAT
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@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]
 

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Let me take a crack on getting some help for my P0420 code, as my Forester is very new to me, just 3 weeks. It's a 2010 non-Turbo and just had both head gaskets done just a couple weeks ago. The HG leak appeared to be oil only, dripping. I saw no signs of water getting into combustion. The plugs, while very worn and large gap, did look a bit dark, but more of a rich mixture than heavy oil. I wish I would have kept them, but they have been thrown away.

I put about 400 miles on it before HGs repair and had no signs of the code being thrown. It only started throwing codes after the HG work, as the ECU relearned. Once ECU was done and running smoothly again, that's when the P0420 threw. I cleared it and headed on a cross-country trip TN > Utah. I got about 800 highway miles before it threw code again. Through the drive to Utah and back, it threw the code multiple times, mostly the pattern was 800 or so miles of highway, then it throws the code when slowing down, maybe through construction, or driving through a town to find dinner. Once the code was thrown, it was not uncommon for it to throw again within a tank of gas. But once back on the Interstate, we would go another 2-3 tanks before the code would throw again.

Mileage seemed as expected: never lower than 23mpg (overloaded vehicle weight, 80mpg, climbing and fierce WY headwinds) and often up at 28 mpg (tailwind, 73mph).

I will say the tailpipe seems sooty. I wipe my finger in there and it comes out black. The plugs were new before my trip, so I could pull them and see how the color looks now.

I did watch the oil and oil level would be fine and steady at full for 1,800 miles, then BOOM, down a full quart. I questioned my sanity (easy to do when driving straight through), and kept watching it. On the return I had the same thing, full and steady oil level, fuel tank after tank, then suddenly down 1 quart with no indication of why. That may be using 4th during steep mountain descents to engine brake ? No clue, but I'm not happy about that oil usage.

My honeymoon with this vehicle is not going well. 'Would love some help on this issue to keep my costs down. My muffler shop is ready for a $750 front cat replacement. I'm concerned about the oil consumption and if this P0420 is not the cat. I've spent too much on the head gaskets. All help welcome. I want to love this Forester!

Thanks- Jeff
 

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2007 Forester Sports XT 4EAT
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@earlyrun the oil has to be going somewhere? If it's not leaking out, then it's being burned, which could overwhelm the CAT = P0420 CEL code.

It only takes a pinhole in the exhaust to trigger the P0420 CEL code. I'm assuming you've looked for leaks.

It's possible the CAT was compromised when the head gaskets were failing, especially if you were getting The P0420 CEL code prior to the head gaskets being replaced?

As to a replacement CAT, looking on RockAuto.com, if you DIY, looks like you can save some money.

Bobby

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

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Thanks for the input sleeper. I had no code prior to head gasket work, but then I only owned it a week prior as well. There may be a leak I haven't caught. This weekend I can get serious about investigating: leaks, condition of plugs to indicated oil burning, if I want to tackle Cat swap myself. Thanks for your reply. - Jeff
 
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