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2009 Forester 2.5 Premium Crappy 4EAT
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Discussion Starter · #1 ·
2009 2.5 4EAT, Premium, 135000 miles. Ive been getting a P0420 for the last year. Even after a new exhaust was installed, including new sensors. Sadly, for me, I just reset the code each time it occurs as the car generally runs "OK". I drive about 70 miles each day (35 miles each way) for a commute that is broken up about 30% highway (65-70) and the rest back roads (45-55). The fuel economy has been anywhere from 22 to 26 fairly steady, but generally speaking it usually stays in the low end unless you really baby the gas pedal. Once in awhile I have had some bad misfiring on #4, tossing a P0304 code. A mech told me it was strange as even with new plugs, wires, etc. it was still occuring, then suddenly after swapping wires between 3&4, it cleared up. Recently this has come back in wet weather. I just put up with it as it tends to go away after a few miles. AS for the P0420, I have had a few mechs tell me that since I have a newer cat, it is most likely a bad sensor. Last weekend I replaced it.

Monday drive all was well. Tuesday, massive misfiring, it was raining like mad. Wednesday a bit better, Thursday fair running. This morning I refilled, and the morning commute was fine, averaging nearly 27 mpg. On the way home, it got weird. Suddenly I get the flashing CEL, AT TEMP, ABS, Traction Control, etc. I have seen these before, but this time the AT TEMP light flashes red then blue. I scanned it and came up with at first the P0420. I reset it and the lights remained. I scanned it again and came up with the P0600. ONce this was done I noticed something strange. The car was acting like it was in one gear, and the fuel economy was increasing. I drove about 10 miles this way, running in one gear at about 3000-4000 rpm. fuel econ at start was about 27, at stop it was over 38mpg (never have I seen it on average above 29). I stopped, and shut off the car for a few minutes, then restarted. all was clear. The car shifted fine and ran like normal. Fuel economy stayed high which was very odd for this car. AFter much start / stop / on / off the econ was around 28.5 when I shut if off. I really do not know what to make of this. I was going to look at the misfire issue again this weekend (check plugs, wires, coil, etc.). Oh, for those unfamiliar, the P0600 is a serial connection fault. I am still researching that.
 

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2001 Forester S, 4EAT
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Regarding plug wires, what brand did you use? Quality is critical to not having missfire. NGK are the only ones to consider.

Sent from my Pixel XL using Tapatalk
 

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2009 Forester 2.5 Premium Crappy 4EAT
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259 Posts
Discussion Starter · #3 ·
I really do not recall the wire brand. I do know the plugs were NGK. The only other thing that was done prior to this, was that I straightened out the routing for wires for Cyl 2 & 4 as these were criss-crossed and not in the wire holders (sloppy mechanic work from a previous visit). Yesterday I checked that all wire connections were sound (sadly did not have time to pull plugs and inspect, etc.). This morning on the commute, I did not get a CEL, however it was missing a little and sure enough, there was a P0304, which has been the case lately. I do think I will do a replace on the plugs and wires and see what it gets me. Also considering the coil pack.
 

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since it happens more with wet or high humidity, start with the wires being tight and wtaer proofed and the MAF and any MAP sensors, a dirty or bad MAF sensor will act up when ot os wet or very humid, and check ALL wire connections, especially fro MAF and MAP and the ECU subarus are known for bad/.loose/.corroded or wet weather subjected connections, check, clean them all, and consider that you are getting bad gas too, add a good cleaner to the gas tank, B12 or mmo or Techron/.regane with PEA
 

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2009 Forester 2.5 Premium Crappy 4EAT
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Discussion Starter · #5 ·
Thanks for the reply. Hoping I can get this Sunday to have a "Car Date" and spend some quality time together.... I am going to get plugs and wires as they are due. Debating the coil as well, but I want to investigate that further.
 

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2009 Forester 2.5 Premium Crappy 4EAT
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Discussion Starter · #6 ·
So, just a quick update, with not much to update. Still getting the daily P0420, thought the last two times, it did not create a CEL. It was only evident when I plugged my scanner in after getting a blinking CEL for a misfire that went away. Anyhow, it seems to be running has it has for the last several weeks / months / etc. Hoping to get some resolution this weekend. I am seriously thinking I have an exhaust leak as I think I can hear it. So a "thorough" exhaust inspection will be on the agenda on Sunday.
 

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2009 Forester 2.5 Premium Crappy 4EAT
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Discussion Starter · #7 ·
This weekend did some work, now to see how it performs for the week. Here is what I did:
-Totally disassembled and cleaned the air intake, including the MAS, back to the throttle body (did not remove that, wish I did).
-Replaced the intake air filter (noticed for the first time the permanent filter in the housing. Curious if anyone has ever replaced this with good results)
-Removed and replaced the plugs (they did not look super bad, but nearly 2 years old...)
-Removed and replaced the wires (did not have time to study them for signs of arcing, etc.)
-Removed and cleaned the coil

Test drove it for about 2 miles on highway (only doing 65) then back roads home (45). Reset the odo (B) and saw the mpg climb to 35, then stabilize at 33 on the hwy, before dropping to about 28 on the 2-lane rd. This morning on the commute to work, with fresh full tank of gas, I saw a high of 27 mpg. More importantly was that I did not encounter any stuttering, or hesitation at all. It did not throw the P0420, but I expect that this afternoon. I did not have time to really look at the exhaust for a leak.
 

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‘14 FXTT; '15 Legacy FB25
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Where is the ECM on these?

Would it help to disconnect, clean, add some dielectric and button'er back up?

I'm not familiar with PO600, but a quick Google search doesn't show it being related to anything you're touching/replacing/cleaning.
 

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Discussion Starter · #9 ·
Just a quick follow up to this. After my last post, I never had another code, with the exception of the P0420, which I get constantly. I have an OBDII scanner plugged in 24/7, so just reset it as it pops up. I am since the 02 sensor is new, and appears to be functioning as it should, I seriously think that I killed the cat by driving too long with the misfire. Last weekend, I installed a spark plug non-fouler on the rear 02 sensor. This was done solely to get me through the inspection process. Once inspected, I have time to consider what I want to do; new cat, new car. The car is nearly 11 years old, 150000 miles and gets meager gas mileage. Once I get it inspected, I will remove the non-fouler, since on the 05+ cars, the O2 sensor also has impact on fuel trim and I certainly do not want any more problems.
 

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I seriously think that I killed the cat by driving too long with the misfire.
You may have but you don't want to just swap parts. That gets expensive.

Get a non-contact thermometer (one of them things with a laser pointer attached) and measure the temperature going into the CAT and out of the CAT. You'll want to get onto the pipe, not in the heat shield.
The outlet should be significantly hotter than the inlet (by easily 100 degrees).
If it isn't, that CAT lost its ninth life.
 

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Discussion Starter · #11 ·
Yes, I agree, and actually a good excuse to buy a new tool, lol. Since it was an aftermarket cat and I ran it very rich for awhile, I will not be surprised in the least if it's cooked.
 

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Yes, I agree, and actually a good excuse to buy a new tool, lol. Since it was an aftermarket cat and I ran it very rich for awhile, I will not be surprised in the least if it's cooked.
:)
Yeah, that's me too. I need very little excuse to buy a new toy.. um.. tool.

If it is a bad CAT you may notice the car bogging down at some point, not wanting to get out of its own way. I'd fix it now, while the outside temperature is still reasonable and you don't have to lay under the car on a cold cement floor.
 
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