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2016 Subaru Forester XT Auto CVT
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Discussion Starter · #1 ·
Currently my FXT is at the stealership, with them attempting to fix it. It was in the shop last week for 3 days and they diagnosed it as having a bad MAF sensor. They ordered a new one and replaced it. I picked it up on Friday and it seemed like they got it figured out, car was driving fine but I didn't put many miles on it that night. Stopped by the store and went home so maybe 5 miles total, but it seemed to be back to it's regular drivability. Not so come the next morning. I started it and it was really sputtering, plus hesitating when trying to accelerate. After about 10 minutes of driving, it thru the P0172 code again. Since it was on a Saturday I did call Subaru, but they didn't have a tech to check it out, so I agreed to bring it in on Monday. Monday came and I managed to limp it to the shop, and it's been there since. I am only writing this so that maybe, if and when it gets fixed, I want to relay to others what the exact cause was. It's under warranty so hopefully whatever it is will be covered. I have scoured the internet on this issue but came up with nothing. There was a similar post on this forum but it was a dead thread, so knowing a little about cars I thought I would share and maybe bring things to light for someone else possibly. Meantime I am driving a 2019 Outback limited on the stealerships dime, so I am not down and out.
 

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2017 Forester XT
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27 Posts
My 17 XT is throwing the P0420 code as well. Had my dealer take a look and all they did was reset the code. Then it came back the same evening I picked the car up. It’s going back in this coming week. My car runs fine at the moment, and I’m monitoring my fuel consumption just to get a sense check.

Let me know what your dealer finds, seems the code could come from a bunch of different things. I’ve eliminated an intake leak. It might be an exhaust leak, or one of my O2 sensors, though seems unlikely it would go at 20K miles.

I’ll post if I have an update.


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2016 Forester XT
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221 Posts
I would suggest getting your battery tested. Sometimes a failing battery will cause sensors to trigger codes when they're not getting the voltage they expect. It's quick, cheap, and easy.
 

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2016 Subaru Forester XT Auto CVT
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11 Posts
Discussion Starter · #4 ·
Well the dealer has had it for over a week now and they are going to replace the O2 sensors, and go from there. As far as the battery goes, I replaced the stock one with an Optima back in December. My guess is that it's a sensor issue, but I will keep updating as things come to light.
 

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2016 Subaru Forester XT Auto CVT
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11 Posts
Discussion Starter · #5 ·
Okay....so down the rabbit hole we go. Week 2 at the stealership, and got a call last night from the service manager. He stated to me that they found "scotch locks" on the wiring harness and that SOA will not warranty that. Good thing I have an aftermarket warranty ;) Well my first thought was "why are those there?"... because I know what they are used for. Generally for tapping into power for additional gauges or troubleshooting a problem. I can honestly state that I didn't do that. I bought the car used last year with 36k miles on it and it seemed legitimately stock. If anyone taps into wiring it's usually because they want auxiliary power for something. It doesn't make sense to me, because the XT already has a digital boost gauge and anything you want to know is already right there on the info screen....ie...temp, acl, mpg..ect. Here's the funny part, even after replacing the MAF, and 02 sensors, they don't even know that the "scotch locks" in the wiring harness are even the issue! This is all strange to me. So I got them the extended warranty info and they will continue on with it. Apparently they have over 8 hours into looking at it, without a clue what the exact cause is. Worst case scenario...I limp it over to another shop and see what they can do, and social media blast our local Subaru dealership. I will keep updating.
 

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2017 Forester XT
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27 Posts
My dealer is replacing the catalytic converter. I should have the car back in a couple days. They aren’t replacing any sensors as far as I know. I’ll update again when I get the car back.


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2016 Subaru Forester XT Auto CVT
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Discussion Starter · #7 ·
The dealership replaced both the MAF sensor and the O2 sensor. I picked it up on Friday. There was no CEL on, so I decided to go get a second opinion on it. Took it to a local Subaru guy, and he couldn't find anything wrong. With no CEL on, he was unable to diagnose it and said go drive it and see if it will throw another code. I put 125 miles on it and it seems to be back to normal.
 

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2017 Forester XT
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Got mine back after the dealer replaced the catalytic converter under warranty and no codes for a couple days now. I didn’t have the P0172 code, just the P0420 code. Dealer said all my sensors checked out.




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2009 Forester 2.0 XS
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403 Posts
In the balkans I used to get P0420 all the time with my old impreza. Most probably due to the lead in the fuel.
 

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2016 Subaru Forester XT Auto CVT
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Discussion Starter · #10 ·
Well so far over 400 miles down the road now and no more codes...wood "knock,knock" But other than being a little rough on a cold start, it has been good. What's really strange is that if the dealership corrected the issue by replacing the MAF and O2 sensors, why didn't they realize that it was fixed? Maybe a failure to communicate between the tech and the service department.
 
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