Pending P0171...what to do?

[nlitend1]

My 2005 Outback XT has had the check engine light come on randomly 3 times in the 2 months that I've had it. When I had it in the shop for the timing belt, they said that it threw a CEL saying it was lean and said they removed the MAF sensor and it was "filthy". They asked me if I had recently re-oiled the K&N filter that was in it. Of course i had not, but remember the previous owner had installed it about 60k ago. 1k miles ago I replaced the K&N with a purolator paper filter and haven't had the CEL come on since. I recently bought a scangauge and now i now see that sometimes i have a pending code of P0171.

Where to go from here?

Option 1 - don't worry about it and simply reset the CEL (if it comes on) with my scangauge.

Option 2 - Replace the MAF sensor (unsure if these reading are actually causing a lean mixture and could possibly damage my engine. Also, could replacing the sensor possibly increase MPG's by providing more accurate readings.)

Option 3 - Replace the fuel filter or run some fuel injector cleaner. I just found out that this is located in the gas tank, so that doesn't sound like much fun... but more than likely it's never been done and the car has 106K on it now.

Optioin 4 - Other. Insert Your recommendation here.

nlitend1

[nlitend1]

UPDATE: Pending code has tripped CEL. I reset it with Scanguage and it's now pending again. Does anyone have any thoughts on this situation?

Thanks in advance,

nlitend1

[skshepherd]

Well, I had the same code for the opposite bank p0174 and the gmc dealer where I bought the car used, replaced the o2 sensor. I read many places on the net that that would most likely not cure the problem. Anyway knock on wood, its been a little over 100 miles and the cel has not come back on. I was sure mine had a leak on the o rings on the intake manifold. Good luck.

[Climber2377]

maybe this will help you? P0171 - Check Engine Light Diagnostic Trouble Code Description

[nlitend1]

Update: I decided to not replace anything regarding the O2 sensors or MAF sensor as this light was only coming on once every 4-6 months and seemed to only ever happen during extended idle. Since i drive 150 miles of freeway driving daily, it has not been an issue.

As stated before, I have a scanguage and was easily able to reset the CEL without issue. The only problem i have noticed is that occasionally during a long extended hill climb on the freeway in 5th gear, I get some hesitation. If i downshift the hesitation stops. On the flip side, if i push the accelerator forward it throws a Cylinder 1 misfire code. The car now has 160k on it and has had a full-tune "major service" tune-up at 120k.

Here in California we have a smog check requirement every 2 years. I just came from the smog check place and unfortunately FAILED. The reason for the fail was that a check engine light came on (the same P0171 code). It passed the emissions part of the test easily, but i'm wondering if the extended idle of the test/a pending code in the system may have triggered it. I have a hard time believing it would be the extended idle because i have idled for a long time in a traffic jams without the light coming on.

I'm kind of in a bind here as i need to renew my registration by next week in order to update my tags. (long story behind why i waited until the last 2 weeks before it was due, but regardless...) My first thought is to replace that MAF sensor that has the above history of being coated in oil from the K&N filter. Is that logical? If the sensor is bad, could it cause this rare trip of the code? If that doesn't fix, it I'm dealership bound i think...

Again, any thoughts?

P.S. Again, thanks to the guys who have already replied! I realize it could be a number of things that cause this generic code..but i'm hoping to get some ideas/experience from subaru owners on this issue.

[cardoc]

Vacuum leak. It could be an intake seal, hose cracked/loose, injector seal or a gap in the intake tube between the MAF and throttle body. Any air entering the system after the MAF will cause the math to be off and you get a lean issue.

[nlitend1]

Originally Posted by cardoc

Vacuum leak. It could be an intake seal, hose cracked/loose, injector seal or a gap in the intake tube between the MAF and throttle body. Any air entering the system after the MAF will cause the math to be off and you get a lean issue.

Thanks for the reply. Any tips on narrowing down this broad code?I changed the MAP sensor and I don't think that anything is very different regarding the hesitation during an uphill climb. There is still no CEL. I might just take it to the dealer, but I'm very worried that will just start replacing parts and rack up a huge bill for a problem that isn't really affecting operation.any tips to help the narrow it down efficiently? Could a bad PCV valve cause this? I'm not sure if t has ever been changed. I'm not burning oil or anything..I'm only down .maybe 3/4 quart every 3k miles. The tiurbo on this makes it hard to change, but I might look into doing that anyway due to high mileage. I heard that PCV valves affect acceleration if they're dirty/bad, but the hesitation only happens during freeway speeds and uphill climbs. The only thing I noted is that at idle, if u just push the accelerator slightly the RPMs drop slightly before accellerating.

[cardoc]

A clogged PCV valve can cause issues. Its a restriction in the engine's ability to breathe.

Check all the joints in the intake tubing from the MAF to the throttle body and make sure none of them are loose and have a tight seal.

If you have a spray can of brake cleaner or WD40, you can run the engine and spray the base of the injectors, seal areas at the base of the intake, the vacuum hoses, joints in the intake tubing from the MAF to the throttle body and when the engine speed changes, the point you sprayed is the leak.

If you take it to the dealer, they will pressure test the intake system to check for leaks or use a smoke machine.

If there isn't a leak, the fuel system needs to be checked for pressure and flow.

Another optional issue would be dirty injectors, carbon build up on the throttle body, or a slow AF sensor.

[nlitend1]

Originally Posted by cardoc

A clogged PCV valve can cause issues. Its a restriction in the engine's ability to breathe.

Check all the joints in the intake tubing from the MAF to the throttle body and make sure none of them are loose and have a tight seal.

If you have a spray can of brake cleaner or WD40, you can run the engine and spray the base of the injectors, seal areas at the base of the intake, the vacuum hoses, joints in the intake tubing from the MAF to the throttle body and when the engine speed changes, the point you sprayed is the leak.

If you take it to the dealer, they will pressure test the intake system to check for leaks or use a smoke machine.

If there isn't a leak, the fuel system needs to be checked for pressure and flow.

Another optional issue would be dirty injectors, carbon build up on the throttle body, or a slow AF sensor.

Thanks for the reply!

Since this is the XT version, taking the PCV valve out to check/shake it is quite a chore since it's located under the TMIC. Any tips for signs that it might need replacing before i go "digging" for it? I posted about some hoses/PCV info on the following thread...Name that hose...PCV valve? 2005 Outback XT

Anything sound familiar there?

Great idea for the brake cleaner/WD-40. Although since everything is so compact with this turbo'd engine I'm worried i'll have trouble isolating a leak....worth a try though..thanks! Since this light only comes on once every 4-6 months or so, do u think it's likely that there is a leak that would be found using this method?

[cardoc]

Use the straw for streamline spray. I have seen some cars with loose or cracked intake tubing pop up a 171. Again, any air entering the engine post MAF will be considered a vacuum leak since it is air that is unaccounted for and causing a lean condition.

It could also be a lazy AF sensor. Pin testing or a scan tool with data will narrow it down if you don't find a leak.

RomRaider data logger is free and PlainOM posted on P0420 Diag that the VAG-COM KKL cable works with the software and is relatively inexpensive.