Subaru Forester Owners Forum banner

801 - 812 of 812 Posts

·
Registered
Joined
·
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...
 

·
Registered
Joined
·
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!



Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
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!
 

·
Registered
Joined
·
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.
 

·
Registered
Joined
·
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!


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
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!
 

·
Registered
Joined
·
2 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.
 

·
Super Moderator
Joined
·
35,216 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]
 

·
Registered
Joined
·
2 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?
 

·
Registered
Joined
·
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
 

·
Super Moderator
Joined
·
35,216 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]
 
801 - 812 of 812 Posts
About this Discussion
811 Replies
215 Participants
2.5x_sleeper
Subaru Forester Owners Forum
We’re the best Subaru Forester Owners Forum to talk about the best years of the Subaru Forester, modifications and reliability history of the Subaru Forester.
Full Forum Listing
Top