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2011 Forester Automatic
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Discussion Starter · #1 ·
Hi. It's my first time posting in the forum, but I have used it many times before to learn more about my Forester.

I own a 2011 Subaru Forester 2.5X Premium with 52,000 miles. No problems thus far aside from a valve (cover?) gasket covered under warranty... quick fix around 42,000 miles.

This past Saturday after driving back from skiing, the check engine light came on while in park and the hazards on. Check engine light, blinking cruise light, and steady traction light. From what I understand, those features are disabled when the check engine light is on. For some background, about 2-3 weeks prior, I had noticed a decrease of about 10-15% in average fuel economy. Also, I'm at about 5K between oil changes so it's time for another one. Finally, I haven't changed the spark plugs yet.

I took the Forester over to AutoZone to check the codes. It came back P0171 (three different P0171 codes). Here are the details:

Troubleshooting P0171
System too lean - bank 1 (cylinder number 1 denotes bank 1)
Probable Cause
1) Faulty MAF Sensor
2) Large vacuum leak
3) Low fuel pressure
4) Plugged injector or cylinder misfire

Any ideas as to what the problem might be? I have an appointment at the dealership for an oil change and engine check tomorrow (Tuesday) so I'm interested in getting some ideas beforehand. The autozone rep said it could be the oxygen sensor, but discarded that idea after getting the printout. Said it could just be a dirty injector. He also tested the battery - gave back a "100% charge" but a "bad battery reading".

Any help would be appreciated! Thanks in advance.
 

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2011 Forester Automatic
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9 Posts
Discussion Starter · #2 ·
Also, I have the Subaru Gold Plus extended plan. Would a solution like cleaning or replacing the MAF sensor be covered? Service to clean the fuel injectors, etc? I'm trying to find a comprehensive list of coverage, but I supposed I'll just have to check my contract (in the glove compartment).
 

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2011 Forester
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14 Posts
Hi CO Forester. I am getting that same code and was wondering if you ever found out what caused it. My Forester is out of warranty and I am reluctant to bring it to the dealer. Thank you.
 

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2012 XT Touring 4EAT
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3,751 Posts
Hi. It's my first time posting in the forum
Welcome aboard! BTW, what model Forester do you have?

Here are the details:

Troubleshooting P0171
System too lean - bank 1 (cylinder number 1 denotes bank 1)
Probable Cause
1) Faulty MAF Sensor
2) Large vacuum leak
3) Low fuel pressure
4) Plugged injector or cylinder misfire
If you are still under warranty, let the dealer sort it out. But talk to the mechanic that did the work and try to determine just what the problem really was.

The troubleshooting info is actually pretty good. You (or the mechanic) might try the following:

For the MAF sensor, it might be faulty, but it's probably just dirty. It comes out with 2 screws - lifts straight up - the gasket may stick. Unplug the connector. Spray the MAF with brake cleaner - blast it good, up inside, etc. Installation is the reverse of removal. BTW. if you use an oil-wetted air filter (like some K&Ns) too much oil can easily trigger this code.

Vacuum Leak(s): Look around your engine compartment for loose vacuum hoses, cracked or brittle hoses, etc. Look at the PCV hose - tight fit? Look at the hose from the intake manifold to the brake booster. Tight? No cracks? Foresters have a rather elaborate vacuum gallery and manifold so just check it hose by hose. Standard automotive practice for vacuum hoses is to use non-reinforced non-clamped rubber hoses so that the elasticity of the rubber makes the seal. Almost anything you do to change this will be an improvement.

Low fuel pressure could be caused by a clogged fuel filter, but filters usually don't give that much trouble. The filter is inside the gas tank - a PITA to replace. But you might be able to do some good with the next step.

Plugged injectors could be caused by lots of things - dirt in the fuel, crud build-up, simply too much commuter service for an engine that wants to run. Try running a can of Techron or SeaFoam or MMO through the gas (which may also help the fuel filter). You could also remove the air box and clean the throttle body with an aerosol spray. Or, you could find an open road and give it an 'Italian tune-up'. Or, do all of the above.

Final thought: For your year vehicle, plugs should still be good. Subaru claims they are good for 60 months, as I recall. Personally, I would change them. Plugs are relatively inexpensive but drastically effect running. On the downside, they may be a nuisance to change (they're hard to get to). With your mileage, I would be thinking 'spark plugs'.

Good luck. Post your results or outcomes.
 

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1998 Forester
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951 Posts
OK, need to post info on the dreaded p0171 code for future seekers.

So the car "sees" a 0171 because the fuel trims (learned fuel values) are too high. On a generic OBD diag tool that lets you see live data, you look for Long term fuel trim and short term fuel trim and add them together. Short term can dance around so use a average, and if you have a positive and negative numbers apply appropriately. IF the numbers are above 14% it is getting too high; but the last two I had numbers combined were pushing 50-60% hence the cel.

If there is an air leak past the MAF sensor than the un-metered air is seen by the Air Fuel Ratio sensor and it ups the fuel trims. that wont solve it so it ups them some more till eventually it sets the light.

It can be a bear to find the issue, two I did last year were vacuum leaks- one a Tribecca had a bad egr valve, the other I didn't get a gasket on the egr tube correctly. As a tech we want to be able so shortcut the long drawn out procedures if possible and quickly find the issue- we earn more money being fast..

When you have a code, you must determine one of two things. Is the sensor lying or telling the truth. Is the code correct, do I have a lean condition, or is a sensor causing the lean condition. The air fuel ratio sensor can both cause and lie at the same time.

The first car (2011 forester 79,000) came in on Thursday at about 330; 1/2 hour from quitting time with no appointment. I scanned it and saw the codes and noted the fuel trims. IF you reset the Cel, the trims will return to zero and you can watch the numbers start the climb back up; indicating a true problem; and this car did this. I test drove it and saved the data, had the shop Guru look at the a/f ratio sensor data; based on it the sensor was operating correctly. I spoke with the customer and informed him I would need more time to properly diag his car, so he said he would return next week. The next day another 2011 Forester 34,000 mi. came in with the exact same code at 3 pm! Sheesh! well at this time I had some time to roll the code around in my head; checked my junk cabinet and found a a/f ratio sensor with he note on the box - 2011 forester, possibly no fault- we acquire these when we are trouble shooting a car and don't solve the issue- this was off one of the air leak cars. Put it in the car and immediately saw the fuel trims correct themselves (long term was +29 and short term went to -29, end result "0". replaced the sensor with an new one and sent the car out, called customer number one and had him come in Saturday- repeated the test; new sensor and two cars fixed.

So even with the sensors "looking" good they were the cause of the issue and both cars had low mileage. I did find it ironic that they were both 2011's
 

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2011 Forester Ltd
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2 Posts
@Sackett (Steve),

Thanks for the informative post. I've been chasing a P0171 in my 2011 Forester for a few thousands miles now. Based on the service manual, I set my sights on the MAF, intake and exhaust leaks, air temp sensors, and fuel pressure. I cleaned the MAF twice, found a leak in the intake, and just today replaced the fuel filter; air temp sensor seems to be working fine. System still too lean. I was thinking about replacing the MAF, but now you have me wondering about the A/F ratio sensor.

I'm guessing it's one or the other, but I would like to narrow it down - I don't want to replace both ($$ reasons). Do you have any tips on assessing the function of the MAF? At idle, it falls within the parameters given in the service manual (0.26-0.66 lb/m). On acceleration, though, it's all over the place. Have you replaced many MAFs in these vehicles?

Or, is the A/F ratio sensor emerging as a more likely culprit in these cases?

Thanks in advance for any additional info you can provide.

One more question I forgot to ask.. I assume you were referring to the upstream A/F ratio sensor??

Thanks!
 

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2011 Forester
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4 Posts
Does anyone have any further insight on this. I've been running with the check engine light coming on and going off again for a few thousand miles now. Code always reads as P0171.

So far, I have changed the MAF, had a vacuum leak/smoke test done, and changed the spark plugs (at ~70k miles). Code still popping up. The dealer here in Tampa wants $70 to do a diagnostic, which I'm assuming means reading the codes and guessing what part to change next.

The only symptoms I'm seeing are reduced fuel economy and on cold mornings (~60ºF - Florida cold!) seeing slightly sluggish initial acceleration for the first minute or so (while the cold engine light is still on).

Any advise on what I should do next??? Really trying to same some $$$ here.
 

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2011 Forester
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P0171 seems fixed for me

I struggled with this code for a while and I ended up replacing the AF sensor with an OEM replacement I found on Ebay. I've driven about 600 miles since I changed it at highways speeds, as well as stop and go traffic. I had several mechanics look at the car and everyone was saying its the MAF or a vac leak but cleaning the MAF and smoke testing it didn't help us find the problem. One mechanic said "check out your tailpipes, they are black with soot". He said this indicates the car is running rich. But the AF is telling me its running lean! I came across this forum and decided to try the AF sensor since it seemed to be lying about the car running lean. @Sackett from Oregon made me feel much more confident and trying the AF sensor replacement. Thanks for the informative post, people from Oregon are the best! :smile2:

One more note I wanted to add. I noticed my PCV valve seemed to be sucking up oil. I replaced this with an OEM part first before the AF sensor since its a cheap part. This did not fix the issue, even after extended driving. My emission readiness monitor never fully cleared until I did the AF sensor. I'm curious if a bad PCV valve could cause the AF sensor to become damaged and complain about a lean condition.
 

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2011 Forester 2.5X Prem
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waiting for lights to reset

Just replaced my upstream O2 sensor in my 2011 Forester 2.5x. So, how long should I expect it to take for the CEL, "traction", and cruise control lights to go out? IE, how long should I give it to run an evap test?
Thanks!
 

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2011 Subaru Forester
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My 2011 Forester 2.5x also throws P0171, it's 75000 miles. The gas mileage dropped and I may smell stronger exhaust when I get off the car sometimes.
I have cleaned/replaced MAF with no luck. (I should have done more research, the AutoZone report said MAF and the guy there suggested clean then replace.)
Now I am equipped with an OBD scanner and service manual.
I have cleaned TB briefly but will take it apart to clean when my new gasket arrives. I have a 3M fuel system tune up kit and used the fuel inject cleaner already. Will use the other bottle to clean my engine gunk when I clean the TB again.
It should not be vacuum leak since the short term fuel trim climbs to 35% for both 0 and 2500 rpm when i read live data with OBD scanner.
I will replace the spark plugs since it's slightly overdue. It's most likely the upstream O2 sensor but I have a question to ask on the service manual:
1.CHECK EXHAUST SYSTEM. (did not do that, any particular place this refers to?)
2.CHECK AIR INTAKE SYSTEM. (tightened a few things and from the SHFT at 2500 rpm it's not likely)
3.CHECK FUEL PRESSURE. (skipped, need a mechanics to do this for me)
4.CHECK ENGINE COOLANT TEMPERATURE SENSOR. (Is the engine coolant temperature 75°C (167°F) or higher? YES, go to 5)
5.CHECK MASS AIR FLOW SENSOR. (Is the measured value 2.0 — 5.0 g/s (0.26 — 0.66 lb/m)? YES from OBD scanner)
6.CHECK INTAKE AIR TEMPERATURE SENSOR. (Subtract ambient temperature from intake air temperature. Is the obtained value −10 — 50°C (−18 — 90°F)? YES)
So here is my question, I read IAT from the OBD scanner and it's about 20°C higher than the ambient temperature, but the service manual then suggests: Repair the poor contact of the ECM connector.
Is there a logic behind of this step? My guess is it's the last resort the manual can come up with (though it obviously ignored the front O2 sensor).
Should I do a step 3) check before I go buy an O2 sensor? (it's not cheap). Any idea on how to do step 1)?
 

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<snip>So, how long should I expect it to take for the CEL, "traction", and cruise control lights to go out? IE, how long should I give it to run an evap test?
You may be waiting forever. Get an OBD-II reader/eraser and clear the code. Some CEL codes will clear by themselves, some not, some . . . who knows? It's much better to clear the code by your own action and then see if it comes back.

For all the OPs in this thread, be sure to check your vacuum lines for leaks, hardened hoses, broken plastic pipes, etc. Most vacuum hoses are noy 'clamped' - they seal by elasticity in the rubber hose. Sadly, this doesn't last forever. Even minute vacuum leaks mess up the fuel trim. Replace the vacuum hoses with reinforced rubber fuel line or reinforced PCV line. It's a great DIY project, not much expense . . . the reinforced hoses last a lot longer.
 

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2011 Forester
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p171 code

Hey @fchen0000,

It sounds like you have the same problem I had. Strong exhaust smell could mean its running rich, when in fact the AF sensor (upstream 02) is saying its running lean. If the sensor is faulty, it could be lying, causing the computer to dump more fuel in. See if you can get someone to check your tailpipes with an exhaust gas sensor at a shop. If its showing a rich condition I would think its the same issue I had, bad AF sensor that's lying about a lean condition. I've driven several thousand miles since I replaced my AF sensor, highway, traffic, city etc and the code has not come back on. I'm also getting better gas mileage now. I'm not saying you have the same issue but it sounds likely.
 

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1998 Forester
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951 Posts
Just going to follow up. If you have a p0171 on a 2011 forester non turbo, replace the b1s1. I have fixed countless of this issue. I recently learned subaru has changed the part number. Don't know of this means there was a modification or what
.
 

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2007 Forester Sports XT 4EAT
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Just going to follow up. If you have a p0171 on a 2011 forester non turbo, replace the b1s1. I have fixed countless of this issue. I recently learned subaru has changed the part number. Don't know of this means there was a modification or what
.
b1s1... for the novice, layperson, is that bank 1, sensor 1?

Bobby...

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

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1998 Forester
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Yes. Industry standard. B1 whatever side cylinder 1 is. Than count down the pipe. Mostly we have 2 sensors, the first one is b1s1, the air fuel ratio sensor the second one b1s2 the o2 sensor.

Sent from my SAMSUNG-SM-G935A using Tapatalk
 

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2011 Forester Auto
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Oxygen sensor

After a few days chasing the dreaded P0171 code and cleaning the MAF sensor without success, I bit the $250 bullet and changed the O2 sensor. Problem solved (apparently!) Certainly runs nicer and hasn't thrown a code so far.
My old sensor came out surprisingly easily. Not the easiest thing to get at, but a 7/8 box end wrench went over the plug and on to the sensor body. Just enough room to get about 12th of a turn but I didn't need any kind of extension to break it free, much to my amazement. Lift the car as high as possible, remove passenger side wheel, and plastic underbody piece and go to it. Total about 1 hour.
I'm sure they can be really difficult to remove and I was expecting the worst (I've had one of those weeks!) but hopefully my luck has changed since I finally :smile2:fixed my pesky garage door opener too!.
 

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1998 Forester
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Yes. We are talking the same. 2011 forester+ p0171 code = replace air fuel ratio sensor. (B1s1)

Sent from my SAMSUNG-SM-G935A using Tapatalk
 
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