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Discussion Starter · #1 ·
I have a Subaru Outback 2011 2.5 Limited which has been showing a P0171 Bank 1 System Too Lean code for weeks now. I have tried the following to fix it: replaced fuel pump with OEM Delphi pump/filter assembly; replaced the MAF; replaced both O2 sensors (before and after cat converter); replaced all 4 fuel injectors; replaced spark plugs. But still have the P0171 even though the car seems to run fine. One thing I noticed was oil on the front spark plug on the bank 1 side (left side). I’m considering changing the cat converter itself but I have also read about oil meaning a gasket leak. Also, a lot of guys seem to fix this with a vacuum hose change? I don’t know and I’m not a mechanic. But I am fairly handy and I can usually figure it out and do the work ... I’m stumped and I’m tired of guessing. Any ideas?
 

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You need an OBD2 scanner to get more information. Fuel trims and other readings can point you towards a vacuum leak, exhaust leak, or something else. I spent $23 on a Bafx Bluetooth scanner on Amazon. I have an Android phone and use the Car Scanner app. Long and short term fuel trims should be near zero. O2 sensor voltages should be in specific ranges. I was able to find YouTube videos to diagnose almost everything that could be wrong with my P2096 Bank 1 too lean.


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01 VDC, 05 R Sedan, 06 BAJA EJ257
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You replaced all those parts, you can continue. Only this time verify the problem.

Too lean means the ECM is adding fuel to the fuel trim map in the ROM. This can be a bad sensor, vacuum leak just about anywhere on the engine, or, a bad battery and/or grounding. The latter doesn't allow things to work proper, or, the systems run slower.

First check is the ampere output of the battery when it's cold and the engine not running and no loads. Then conductance through the ground cable from the neg post to the end of the ground cable by the starter. Then check conductance to the engine block and alternator case. You want .2 ohms or less resistance.

Once that is right, vacuum leak check. You run the engine and with a can of brake clean or carb cleaner spray around the base of the intake, around vacuum hose connections, the brake booster, valve covers, PCV hoses and valve under the throttle body and the seems in the air tube between the MAF and throttle. When the engine rpm changes you found a leak.

Then there's the brand of fuel you use. Some fuels are high in water and this increases the oxygen content and the ECM sees it as lean.

The cat is not going to cause a lean code. It can be damaged by a lean running engine over a long period.

For you and anyone else reading, the MAF, injectors, and fuel pump should be checked properly before replacement. They don't fail often.

Most times I see a lean code on this engine, it's either poor electrical flow, dirty fuel filter or a leak around the PCV valve. That hose on the top of the crankcase adaptor that comes up to the air box on the throttle will also leak due to hardening from heat.

And while I'm on the air box, look for cracks in the seams on the plastic case.
 

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Discussion Starter · #4 ·
Ok, thanks. I didn’t do a great job explaining the order of everything. Prior to the P0171 error code, the original fuel pump did fail. So, I replaced just the pump itself; not the entire assembly. That was a mistake. The car ran OK for a week and then it didn’t. So, I replaced the entire fuel pump assembly. Then, the code appeared. Knowing that it was running lean, I knew it either meant the injectors could be clogged from not changing the fuel filter or it wasn’t getting enough air. So, I started with the easiest stuff first. I changed out the MAF, the air filter and O2 sensors. No luck. Then, I moved on to the injectors and spark plugs. (Keep in mind the car has about 190K miles and has never had any of these parts changed.) Still no luck. Obviously, as you said, I’m not suggesting anyone follow these steps to fix a similar problem. Just wanted to get some help. I will try what you suggest and if it’s beyond me; I’ll just take it into the dealer.
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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Not a dealer. Find an indie shop that works on Subies.

Something else that has come up in my P0420 diagnosis thread in past years is one that had an issue with the EVAP system causing a lean condition and the ECM fuel correction was affecting the cat.

Data from the car along with what I posted above is needed to locate the problem.
 

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2010 Outback 3.6R 2014 Legacy 2.5i
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this is what I did when I had the issue.

1 data log it

on a 2.5L MAF should read roughly 2.5grams/sec at idle warmed up no load
verify engine indeed reaches operating temp
commanded vs actual afr should be very close.


2. SMOKE the intake and exhaust and find the leak, since you replaced all of the other parts, my guess is you got a leak...do it when engine is cold and temps are around the same as when you experience the problem. any air introduced into the stream after the MAF will show up as excessive oxygen in the o2 data stream and the computer tries to compensate by adding fuel.

on my 2014 legacy 2.5i I resolved the issue by data logging it and smoking the engine. found intake gaskets leaked when the temps were -10 or lower.
Since replacement, issue never came back.
 
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