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2009 Forester 2.5 Premium Crappy 4EAT
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255 Posts
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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2 Posts
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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Super Moderator
2007 Forester Sports XT 4EAT
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35,937 Posts
@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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326 Posts
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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Super Moderator
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 diagram 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

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