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2012 Forester X MT
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3 Posts
Discussion Starter · #1 ·
So I have owned a 2012 Forester X, manual transmission for about 7 months now.
In that time, I have logged how much oil I had to replace. The rate of oil burning is erratic, but on average burns about 1 quart every 600 miles. I tried using mobil1, penzoil, and castrol brands.

In the last 2 months, at 120k miles, I noticed a loss of acceleration power and chalked it up to the spark plugs and decided to wait until I returned home (I travel for work) for replacement.

2 days ago, I got a P0420 - Catalyst system efficiency below threshold (bank 1) - at 122k miles. I'm wondering if the excessive oil burning is linked to damaging the converter? I'm not too mechanically minded, but I imagine that the burned (unburned?) oil has to go somewhere. If the problem is actually the catalytic converter, and not the O2 sensors, then I will have to get the oil burning problem fixed as well to avoid another converter replacement in the future. At that point I dont think it's worth repairing both the oil burn and CC.
 

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Super Moderator
2001 Forester S, 4EAT
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3,358 Posts
More likely the burning oil contaminated your O2 Sensors. It's common on vehicles that have had HG replaced, the O2 sensors will trigger CEL.
 

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2011 Subaru Forester
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1,610 Posts
To fix the oil burning you would need a new short block. Mine burns oil as well and would have before I bought it two years ago. I replaced the O2 sensors (A/F sensor and O2 sensor) last summer as preventative maintenance and its been beneficial and the engine runs better. With Covid I have not driven as much as in the past, and I have only done 24,000 km on the Forester ( I bought it with 248,000KM on it), but never had a P0420, and based upon my acceleration the cat is fine.
Basically I just have tried to make sure the engine runs as well as possible. New plugs, new serpentine belt, clean MAF and MAP, clean air filter, clean the throttle body, replace PCV valve and hose and clean EGR valve. If your engine is running as good as possible it should go a long way towards getting the maximum service life out of your cat.
Replacing the sensors may fix the code, and give you a better fuel/air trim. If you have to replace the cat you would want new sensors anyway, so personally I think that is the place to start.
 

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2012 Forester X MT
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3 Posts
Discussion Starter · #4 ·
Today, I did the usual topping off of my oil - about .5qts. And the Check Engine/Cruise Control Blink/Traction Control Lights went away. Not sure why, but I am cautiously waiting for the light to come back and hoping that it will never show up.

Maybe the computer read the pressure loss of oil as a catalytic converter efficiency loss? Im not sure.
 

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2012 Forester X MT
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Discussion Starter · #5 ·
Update: 130 miles in after the lights went away.... going downhill again at about 50 mph, the CEL/traction/cruise control lights went back on.
 
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