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2010 Subaru Forester 2.5X Premium
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Discussion Starter · #1 ·
I just bought my 2010 Forester two days ago. It has about 90,000 miles on it. I filled the tank up with unleaded last night, and after driving about 20-30 miles my vehicle dynamic and engine light came on, with the cruise control blinking. I took it to advanced auto, and they ran the diagnostic tool and it read P0420 and he suggested a fuel additive seafoam. I bought and added it, and the lights went off, however, now every time I come to a stop, or almost stop, my dashboard lights up with the AWD, Oil temp, dyanmics, the sport, and another light. They blink on and off. Can someone tell me what may be going on?
 

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'21 Forester Touring (previously '09)
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Doesn't the 2010 require 91 Octane? Our old '09 did, and if we put in 87 we'd get a MIL fairly soon after. I'd maybe drive out your tankful, then refill with 91 and see if it self-heals.

Oh, and make sure the gas cap is tightened well. That'll fire off a MIL too (don't ask me how I know that). ;-)

Mind you, some of the lights you mention seeing shouldn't be related to a MIL. I'd recommend taking to a Subaru specialist mechanic (or dealer). In my experience, for some reason there's something "special" about Subaru engines that your normal mechanic (even some of the good ones) can't deal with.

If you do that, be sure to mention the octane of the fuel you put in, just in case it matters.
 

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2015 Forester 2.5i Limited CVT
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1,500 Posts
P0420 indicates a few possible problems. An exhaust leak, faulty O2 sensor, and the reason they gave you SeaFoam; a bad catalytic converter. The last being the most expensive diagnosis.

I would back to the dealership or an indie Subaru shop. Do you have a warranty on this vehicle? Usually you get 30 days for used.

Someone must have known about this problem
Did you get a maintenance history?
 

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2010 Subaru Forester 2.5X Premium
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5 Posts
Discussion Starter · #4 ·
I bought it used from someone, so no warranty. The nearest dealership is about 240 miles away. I have a folder he gave us that is supposed to be receipts for anything they dis while they had the car. I’m hoping I didn’t get taken for a ride.
 

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2001 Forester S, 4EAT
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Doesn't the 2010 require 91 Octane? Our old '09 did, and if we put in 87 we'd get a MIL fairly soon after. I'd maybe drive out your tankful, then refill with 91 and see if it self-heals.
Subaru in CA/USA have never required anything other than 87 Octane, with the exception of the Turbo XT models that required 91+ Octane.
 

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2017 Forester XT Touring
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464 Posts
Reset and see if it comes back. Fuel up at a different place for a couple of times. If it keeps popping the P0420, the first thing I'd do is the downstream O2 sensor. I've had this occur on multiple Subarus and a new downstream O2 sensor has always fixed it. That's not to say your cat might not be going bad, but a $50 sensor is certainly worth the replacement before doing the cat.

The Christmas Tree of lights that a Subaru dashboard becomes when it throws an engine code is just something to get used to. The worst part of it, in my opinion, is the disabling of the cruise control for minor issues. Shortly after I bought my 2011 Outback, it threw a P0420 code. Every store near me wanted $250+ for the O2 sensor, but I could get the same sensor online for $60. So I waited a few days for it to come in, and had to deal with my 150 mile round trip (almost all interstate) commute without cruise control.
 
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