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2006 Forester 2.5X
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15 Posts
@ryan427, I cleaned my MAF sensor about 5-6 months ago because of an associated MAF code. That code has not ever returned. Car idles smooth as silk, no abnormal odors, runs like a beast (but just got 30 MPG on my recent cross state trip). I am at about 1200 miles since my last P0420 light and the only change is waiting about 10 minutes or so to engage cruise control. I did mention that it consumes oil in an earlier post however I was doing research on my other Subaru and found a video that I learned I actually have an oil leak in my head gasket (oil leak on right side that looks like a steering stem leak at the steering rack). Again, 3/4 quart in 5000 miles isn't a huge problem but it reassures me that oil deposits are probably not contributing to my problem. I have been doing some reading and I think when the cruise is engaged, the vehicle tells the computer it is at a certain speed even when it is shifting up and down the mountain to maintain the set speed. When the ECU does the P0420 survey, it may be catching it in a high RPM, high fuel demand condition which flags a P0420 code...that is just a theory...
 

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2018 Forester Premium CVT
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21 Posts
@seanlivolsi check that MAP sensor and the port in which it sits. Tiny hole inside that port feeds the MAP sensor its pressure. Mine was almost completely blocked by crud. I know it seems weird, but getting a reading on both the MAP and MAF sensors is a good idea when you have an intermittent 0420. This is according to my neighbor/Subie mechanic. Reason being is these sensors are often damaged by cleaning and/or often get lazy over time (apparently the MAF gets lazier than the MAP). The MAP just gets completely caked up with oil and crud which degrades it’s ability to sense slowly over time. Careful cleaning both these sensors. Underperforming sensors can and will cause rich running conditions so I’m told. Btw the Subie dealer only charged me $55 for the diagnostic and showed me sensor readings. Might be similar in your area. Good luck!



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2006 Forester 2.5X
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15 Posts
@ryan427, thanks for that info! I have an update: after 1600 miles the CEL for P0420 came back on again and the cruise was not on this time. I actually think this relates to fuel more than anything else. This time as well as the last time the light came on, it was within 50 miles of a fill-up. The reason why I think that is because for the last 800 miles or so I have been getting exceptional highway mileage; 28.5-30 MPG. This tank I can already see I am down around 25.5 or so, maybe fuel quality has a part in this. I didn't realize my car had a MAP sensor because it isn't where I normally see them on the intake manifold and parts listings don't show a traditional MAP sensor. I did find a sensor on the throttle body housing that Subaru calls "sensor assembly, pressure" so I assume that is it. I will pull that out once I get back from my trip and see what it looks like. I am starting to believe I may just have a Cat that is degraded and intermittently pops a P0420 depending on the fuel quality. When I say "fuel quality" I am referring to the percentage of methanol they so kindly add to our fuel. Recently in my area our fuel quality has been really good because even my bike picked up 3MPG from normal. Also in an earlier post I did say that I had oil consumption, about 3/4 quart in 5000 miles. What I actually discovered is that my right head gasket is leaking oil so most likely I am not consuming any oil thru the engine. I say this because there was question that oil deposits could be effecting the O2 sensor. I am due for a timing belt in about 2K miles. I just may take the opportunity to change the head gasket, timing belt and Cat. With 214K miles, I shouldn't complain. I could live with the leak but since I need a belt anyway, $35 and about 4 hours of work and I can fix the leak too. I will post an update about the sensor I found later this week. Thanks!
 

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2006 Forester 2.5X
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15 Posts
The sensor and port were both very clean, no issues there. The P0420 code is happening more frequently now and under different conditions that I have previously described before. The system may be working as advertised and it may be time for a Cat. 215,000 miles on the original isn't terrible I guess. The car body is perfect and the engine runs great so it is worth the investment.
 

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2018 Forester Premium CVT
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21 Posts
Gotcha. We just replaced our cat hoping it would clear up the slightly rough idle and some stalling. What I should’ve done was a compression test to make sure everything was mechanically good. We’ve got low compression in the driver side cylinders (2 and 4). This is likely what killed our cat in the first place. Probably not your problem as your motor arguably has a more stable bottom end. Hoping you’ll figure everything out!


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2004 Forester XT
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37 Posts
4020 code

I made the grave error of fueling my 2004 FXT(150.000 miles) while the engine was running, all hell broke loose when I tried to drive off, after it all stabilized now I am getting a intermittent 4020 code. At first it would come on after about 60 to 70 miles and now it could go 2 miles or 80 and it comes on. I put the tester on it and the only code I get is 4020. The engine seems to run ok and mileage is good. Is it possible I damaged the Cat with the fueling debacle? I do not think its the rear o2 sensor because I never had an issue before this happened.
Thanks in advance!
 

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2005 Impreza 2.5RS Wagon
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3 Posts
I know this forum should be for foresters but you guys/girls seem every knowledgeable and since my vehicle is pretty similar I figured id ask here.
I have a 2005 Impreza 2.5rs and about 2.5 years ago I got a code for a failing upstream sensor(dont remember what it was), I changed that with a DENSO #234-9122 and the code went away. About a year after I started getting a P0420 code only when I go on highways for about 10m+ (I work 5m away from home so I don't normally get the code). I got my head gaskets changed about 8-9 months ago. but the P0420 still comes back.

I have horrid mileage ever since I started getting a P0420.
I'm about to buy a new cat by Walker #54537 and a new downstream sensor DENSO #2344732. I already went to my local exhaust shop and other than having to replace some exhaust parts after the cat I have no leaks (at least from exhaust headers to cat I have none)

My question is could a leaking head gasket kill my cat or my upstream/downstream sensor. And would running rich cause soot on my exhaust tip and more than usual condensation out of my exhaust during winter ?

Thanks for any help.
 

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2007 Forester Sports XT 4EAT
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37,486 Posts
@Stephane St-louis I see this is your first forum post, so welcome to the forum from Oregon! :biggrin:

Is that P0420 CEL code the only code you have? :confused:

I have horrid mileage ever since I started getting a P0420
If the poor gas mileage started after you got the P0420 CEL code, then it sounds like you're running way too rich.

My question is could a leaking head gasket kill my cat or my upstream/downstream sensor.
Quick answer, yes. Especially if the engine was ingesting & burning coolant.

Bobby...

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

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2005 Impreza 2.5RS Wagon
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Yes it's the only code I'm getting. Since I have an 05 can a dead cat or downstream give me bad mileage or would it just be the upstream one that would cause that?
 

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2005 Forester LL Bean AT
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21 Posts
2005 Forester bought last year for my son who is in school about 3 hours away.
Chasing a re-occurring P0420 code, I have replaced or checked the following:
PCV valve – OEM – original valve was not clogged
Plugs – NKG – removed plugs looked good, no evidence of oil or unusual wear.
Wires- NKG
Front and Rear 02 sensors – Denso
Cat – Eastern – original front cat disintegrated
Air Filter – Fram
Checked the coil pack resistance
Cleaned the MAP sensor
Checked for exhaust leaks, plugged the tail pipe with car on ramps and listened/ felt for leaks.
Checked for vacuum leaks, sprayed MAP cleaner around gaskets, injectors, etc.
The car runs strong, although the A/C compressor drags the idle down a little.
Replaced the clutch and had the A/C serviced at a shop.
The only other thing the car will do is occasionally after sitting overnight, it’ll crank an unusual amount before starting. Maybe coolant temp sensor?
I’ve asked my son to record some live data but being old enough that my first four cars had points I’m not really sure what to look for in the data.
I’m leaning towards replacing the MAP sensor but would like to at least test the one in the car first.
Any help, or suggestions would be greatly appreciated.
Thanks
John
 

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2007 Forester Sports XT 4EAT
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37,486 Posts
Yes it's the only code I'm getting. Since I have an 05 can a dead cat or downstream give me bad mileage or would it just be the upstream one that would cause that?
The poor fuel mileage, P0420 CEL code & sooty tail pipe sound like you're running way too rich.

I can see getting a P0420 CEL code if the CAT(s) are failing, but I can't see them causing the engine to run too rich?

Questions: :confused:

[1] have you verified there are no exhaust leaks near the front O2 sensor?
[2] Did you replace the front, upstream O2 sensor after the head gaskets were replaced?
[3] Is the Subaru recommended scheduled maintenance up to date?

Bobby...

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

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I have a 2009 Forester XT. It has the P0420 CEL. I installed an 02 sensor spacer and cleaned the MAF. Code shows up now and then but for the most part is gone. There is a stinky smell, unique to that car. Almost sweet smelling, it does not smell like a go-kart or non catted car though. Getting about 19MPG
 

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2009 Forester 2.5 Premium Crappy 4EAT
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259 Posts
I have a 2009, 2.5, 4EAT Premium. I have been driving this since taking over from my wife when she got a CRV 2 years ago. We have had the Subaru since new. About a month after I took it over, August of '17, the CEL, P0420 came up, along with a slew of others I have long since forgotten. Long story is the exhaust was shot. New headers, Cat, etc. back to the muffler was installed. All good. Then about a year ago, I start to get the P0420 again. I had an '05 that had the same curse, so I always had my scanner connected so I could check / reset codes as needed. At first the code would show up maybe 1 - 2 x per month. Then it was every 2-4 starts of the car. Now it is constant. In fact, If I get the code right now and reset it, I will get it again within 10 mintues. Not an actual CEL, but on the scanner it will pop up. Then the CEL after another few start / stop cycles. I have proven this many times. About 6 months ago, I developed a bad misfire issue, so I replaced the coil pack, plugs, wires, PCV, rear 02 and cleaned the MAFl. Sadly, this was not until driving it like this for at least a month. Now, I fear I have wasted the CAT. The car had to go in for inspection last month (Sept) and I had recently reset the codes. I was told that it did not pass since when they connected their emissions computer up to it, it said my cars computer was not ready. They said I would have to drive around for another week or so. Obviously I knew why it failed and also that it would fail again. yesterday I picked up the spacers and will try that. Sadly, I am hoping to get rid of this car soon, but I am fearing a ticket due to lapsed inspection.
 

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2001 Forester 4EAT
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347 Posts
My '01 Forester L gets the P0420 code from time to time as well after I rebuilt its engine. I have learned to ignore or simply erase it with the scanner since it doesn't seem to affect anything else in the engine's function either way.
 

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2007 Forester Sports XT 4EAT
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Son just bought a 2005 Subaru Forester with approx 150k miles on it. Within a couple of days he got the P0420 code. We have been researching the issue and wound up here. Just finished reading all the pages in this thread.

First we watched some videos that showed what the upstream (o2B1S1) and downstream (o2B1S2) waveforms should look like – and what they shouldn’t look like. Purchased a FoSeal Wifi OBD 2 code reader and purchased the OBD Fusion app for my iphone. But we couldn’t see any data on o2B1S1. Looking at the available PIDS I saw that we have 02 sensor lambda B1S1, 02 sensor voltage wide band B1S1, and 02 sensor current wide range B1S1 (as well as numerous others). With that I assume the vehicle has a wideband sensor and will not get the waveform as I saw in the video for 02B1S1. So now we are going to set up to log data and get a log we can look at. Note: we did see a 02B1S2’s waveform varying quite a bit but I cannot guarantee we were up to temp – so I want to redo and look at a log with more history.

Another thing we checked was the temperature on each side of each cat and did see an approximate increase on the output side of about 40F. So it seems the cat is doing something – just maybe at a reduced capacity.

Reading through all the post I understand that there are many issues that could cause or contribute to this code and these should be investigated before implementing the “$5 fix”. So, now we want to check as much as we can but I am looking for things to check and how to check with the tools we have.

Things I gathered from the post.
Check gas mileage to see if the car is using a lot of gas
Check for dirty MAF/MAP sensors
Check for dirty EGR valve
Check for exhaust leaks
Check both 02 sensors

Is there specific data I should be looking at in a log that may point to one of the above? As an example, if I had an exhaust leak in front of 02B1S1 would the vehicle indicate a LEAN condition (a high Long Term Fuel Trim number at idle). Not sure what data I should key in on.

Just trying to come up with a plan of attack and understand what data we should take and what we should be looking for to help diagnose this code.
 

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2009 Forester 2.5 Premium Crappy 4EAT
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259 Posts
An update to mine. I installed the non-fouler (only one) on the rear O2 sensor on Saturday. Since then I have logged about 100 miles with no P0420. Previously, I was getting the code constantly (with scanner plugged in, I get code, clear it, it's back in 10 minutes). This morning on the way into work, scanner connected, The first time since I have been driving this car (2+ years), I saw the Green light on the scanner, indicating all is healthy. Yes I know it is not healthy, the non-fouler is only a band-aid to get me through inspection today. I will remove the non-fouler after the inspection so I do not do any long-term damage due to fuel trim issues. Not sure if I will buy a new Cat, or just dump the car for a new one. It's 11 years old with 150000 miles on it and I have no time, nor patience for the DIY stuff anymore.

Oh, I learned something worthwhile on another auto site. Seems a poster was complaining when they went to install their new replacement Cat assy (for use we can get it as an assy, back to the muffler). Theirs did not fit. It was about 2-3" short, requiring a fabbed piece to be installed. Another poster said that is especially true or apparent in California, where emissions are very strict and they want you to use "stock, OEM" parts. The aftermarket parts, although all there, are not a truly exact fit. This was the case with mine when I had it replaced back in 9/18. The assy was about 2-3" short.
 

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2009 Forester 2.5 Premium Crappy 4EAT
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259 Posts
UGH! another update. From the emissions standpoint, I stand by my plan to remove the non-fouler and go back to the constant P0420 once the inspection is complete. Yesterday I took it in for inspection. emissions passed. Sadly, 3 weeks ago when I 1st went in for the inspection, after clearing the code that day, it did not pass. Apparently, they do the emissions first. If it does not pass, they stop the inspection entirely. So, yesterday, I was shocked when I was told it failed as the the control arms were shot (bushings nearly gone, ends worn, etc.). Now I will have the repair bill to consider. I did order a hand-held infra-red thermometer to check the cat temp, just see if that truly is the cause.
 

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@Javan Post what you get for temps, including the vehicle year/model and where you took the temps. Oh, and try to get right on the pipe and not a flange or heat shield. I don't have the exact temps written down and I'm not sure we were "up to temp", but as quick check on the front cat on a 2005 Forester I think we got around 380F on the front side of the cat and around 420F on the backside (output) of the cat. So we did see a 40 degree rise from input to output, but I'm not sure how this compares to other cats.
 
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