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'06 Forester 2.5x throwing P0171 code (merged thread) - Updated!

5992 Views 12 Replies 5 Participants Last post by  2.5x_sleeper
Hey guys. Code showed up, randomly, about 2 months ago. I replaced the air filter, cleaned the MAF, replaced both 02 sensors, PCV valve and hose, plugs and wires, and even ran a smoke test. Now, I did all these things individually, and reset the CEL, after each change. I pulled the main vac line off the brake booster, and ran smoke. Only place smoke came out of, is the intake snorkel. So as far as I can tell, the system is sealed, end to end.

Any other suggestions? This is driving me nuts. There's literally nothing left to change, that would make any sense. The car runs fine, idles beautifully, never stalls, and never hesitates. If it weren't for the fact that it's disabling my cruise control, and costing me 3 mpg, I wouldn't even care.

Any suggestions or solutions would be greatly appreciated.
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The only thing suggested in that thread, that might work, is a new MAF. But i'm hesitant to blow another hundred, on someon ng that seems to be functioning. Like I mentioned, the idle is fine, constant and smooth, no strange oscillation, no stall indications. According to my monitors, both 02 sensors rad right around 14.7, which is normal barometric pressure, so i'm kinda at a loss.
Your O2 sensors do not read pressure, they read air/fuel mixture.

14.7 is right around stoich, but they should be continuously fluctuating.
What does the front read while driving? Cruise, starting, full throttle etc.

Did you use OEM replacements for your O2 sensors? This era Subaru is very picky, even about Denso vs Bosch.

If you've ruled out the air side of the engine, the fuel side is next.
Old dirty clogged injectors, dying fuel pump, clogged fuel filter etc are next.
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Yeah, they were fluctuating, but I haven't looked at them, under driving conditions, just revving the engine in the driveway.

I used Denso OEM sensors, for both. I have some BG 44K on order, to see if it's maybe a clogged injector.
Would be a good idea to pull the fuel pump hanger and check pump/sock/filter. Not a hard job.
Check to make sure the pump is getting a full 12v as well to rule out any wiring issues.

Replacement injectors are $20 a piece as well. Fuel pressure regulator is pretty easy to do as well, and not too expensive IIRC
I'm going run some cleaner through it tonight, and see what happens. If this doesn't do it, i'm going to take it to a shop, and let them run a deep scan on their machine. Maybe they'll find a code, mine wasn't pulling
P0171 - I give up

Ok, 2006 forester x, 141k on the odo. Started getting the P0171. Replaced the air filter, fuel filter, PCV, both 02 sensors, plugs and wires, and cleaned the MAF. Ran an end to end smoke test. ECU shows both O2 sensors working fine, and vacuum pressure holds at 22 Hg, at idle. Reset the ECU after each change, code comes back between 35 and 70 miles, every time.

At a complete loss. Only thing left to do, is replace the MAF or MAP, unless someone has any other ideas.
@Gigatron your threads have been merged. There's no need to start a new thread, when you already have an active thread on the same subject. It best to keep all the trouble shooting history together. :wink:


['07 FSXT MODding Journal] ['03 X MODding Journal]
I'm going run some cleaner through it tonight, and see what happens. If this doesn't do it, i'm going to take it to a shop, and let them run a deep scan on their machine. Maybe they'll find a code, mine wasn't pulling
Did your shop find anything? Curious as to what might be causing your code.
exhaust manifold

@Gigatron Check the exhaust manifold for leaks or just replace the gasket and re torque the manifold. Air is entering the exhaust system and the the O2 sensor reads it as running lean.
I pulled the MAF, all the way out the car, soaked it in MAF cleaner, then when at it with some picks and q-tips. Just had to make sure to take caution with the resistor wires, waaay up inside the housing. You'd be surprised how much crap comes out.

Anyway, reinstalled it, and reset the ECU. So far, it's been about 80 miles of mixed highway and local, and restart cycles. Not showing any pending codes yet, so maybe this is a good sign.

If it works, thes lesson is - don't just spray the maf, pull it out and clean it, so it looks factory fresh. Any luck, I'll get another 140k trouble-free miles, out of it.
This is the instructions given to me by my COBB tuner. He wanted me to clean the MAF before my COBB dyno protune session:
The MAF sensor is very easy to clean. You can use a MAF cleaner (CRC makes some) or isopropyl alcohol (rubbing alcohol). If you end up using a non-aerosol liquid, just pour some into a clean zip-lock plastic bag. Seal the sensor in and shake the bag by gripping the sensor housing. You'll probably be surprised at the gunk that ends up in the bottom of the bag! If it's aerosol, just spray (gently!) inside and out. Make sure you don't poke anything inside, or you could damage the sensor!
I added "Updated!" to your thread title. :wink:


['07 FSXT MODding Journal] ['03 X MODding Journal]
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