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Old 12-29-2003, 02:51 AM   #1
CirrusWRX
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Default P0171 CEL Thread - Everything you ever wanted to know?

More of an informational post than anything else. PAWRX on the boards here recently went through hell (and $$$) trying to diagnose his P0171 - Fuel trim malfunction (A/F too lean) CEL after de-modding from ~Stage 4 setup. With his permission, I'm going to post what he (and I) went through in addition to the stuff I dug up on NASIOC, i-club, ClubWRX, WRXHackers, ScoobyNet, and Google. I will try and list things in the most appropriate order of "what to check first." I'm probably going to miss some things, so please clarify if you have any experiences that contradict what he/we tried, or if I'm flat out wrong about anything. Without further adieu, the

PAWRX & CirrusWRX guide to troubleshooting a P0171 CEL code on a 2002/2003 WRX

First disclaimer: As with any troubleshooting, if you get this code after tinkering with something on your car, chances are the tinkering caused the code! Always look to what MOST RECENTLY changed (which can mean: installed, uninstalled, modified, etc...) when troubleshooting, no matter how remote the possibility may seem.

1. Cold air intakes. Just as the name of the CEL suggests, the ECU is sensing a lean condition. As anybody with half a search function will find, many CAI's for the WRX will cause a lean condition due in part to either relocation of the MAF, a differently sized diameter of piping, or purely increased flow of air which is not correctly compensated for. Many people who get this code get it shortly (or instantly) after installing a CAI. The CAI would be great first place to start, especially if you recently added one.

2. Stock airbox not sealed. Assuming you have a stock airbox, check the connections and tabs around the airbox itself. Many times, people think they "seal" the airbox, but don't actually have the top and bottom parts aligned correctly resulting in a leak. Check to make sure the air filter (esp. if recently replaced) is sitting in the right spot and the clamps are tight and everything is lined up properly.

3. Hose leaks, generic air leaks. Mostly in and around the intercooler, turbo, BOV, vaccum lines, turbo, etc... There is a vaccum hose coming out of a nipple on the right side of the BOV on the IC; the return/recirc. line runs off of the BOV under neath the IC to the intake. It gets pulled off easily if you remove your IC without disconnecting the BOV. There are some vaccum line connections on the turbo inlet hose. The Samco turbo inlet hose does not fit 100% perfect - there could be a small leak or tear. Check the turbo outlet, and Y connections on the IC. Check for a tear or leak or loose clamp on any replacement hoses. Check all hoses in any area where you were recently working on something. Boost gauge installation or UNinstallation. IC installation, FMIC installation/uninstallation. All those hoses need to be securely connected and routed to the appropriate places! If necessary, find a buddy with a WRX, park them side by side and trace all the lines. Chances are you would would also notice some sort of boost fluctuation/abnormality if there were a leak, but this is not always the case.

4. Fuel pressure regulator. According to the factory service manual and suby techs, the most common problem with the FPR is the vacuum connection to the intake manifold (either cracks in the hose where it makes a sharp bend, or a leaking connection.)
Occasionally, the fuel pressure regulator reference line will pop off or is accidentally left off when working on or around items located near the passenger side fuel rail. (ie. injectors, spark plugs, reaching for something...) This is a serious mess up, but one that is easily corrected.

5. Check all ground connections. While you can certainly verifiy all of them, one person mentioned specifically being able to fix their problem by checking their grounds. They discovered that the ground wire on the drivers side of the intake manifold (the bolt) was loose and was causing the P0171. Make sure that bolt, and all others, for that matter are tight. This one is somewhat random, but it's so easy to check.

6. Injector problems. I came across two people who posted about a P0171 after removing AFTERMARKET injectors and going back to stock injectors. In both cases, the stock injectors had been sitting in a box inside their houses for over a year. It was believed that the fuel had crystalized and caused a blockage of some sort on one or more of the injectors. This could potentially happen with ANY used injector that has sat for a long time. In both cases, the individuals used fuel system cleaner (is it called "heet"?) and the P0171 went away.

7. MAF sensor gone bad/dirty. This is an easy one to check if you can find somebody willing to swap MAF's for a day or two. The MAF looked good on PAWRX's car, but we decided to swap it with mine just for gits and shiggles. The process to do this is so simple, requiring a single clip and two screws. Swap MAF's with a local (and friendly) WRX buddy, reset your ECU and drive for a day or two. If you get your P0171 again, the MAF was not it. If you go CEL free, and even better, if the OTHER person gets a P0171, then you found your culprit. Either clean the old MAF or shell out the cash for a new one. In our case, PAWRX got his CEL the same way as always after we swapped MAF sensors.

8. Front oxygen (o2) sensor, located in passenger side exh. manifold gone bad. This was the final solution to PAWRX's problem. Again, we "swapped" (I didn't actually install his o2 in my car) and his problem disappeared. His o2 looked "worn" but not bad by any means. His experience with the P0171 basically had the car run fine for the first 10-15 miles until N.O.T. Then, after power cycling (shut off car for a minute, turn it back on) and driving at a constant highway cruise for a few minutes, the CEL would come back on. After checking #1-#7 above, we were left with swapping the oxygen sensor, and that was the ticket in our case. After trying a few cruises and power cycles with no CEL, he ordered a new sensor from Subaru. While waiting for the nwe sensor to arrive, he reinstalled the "bad" o2 sensor in his car and drove home. Upon starting up the next time, he got the CEL. Since installing the NEW o2 sensor 3 days ago, no CEL.

In his case, with the "bad" o2 sensor, the CEL would disappear after resetting the ECU, but would come back on after shutting the car off and cruising on the highway for a few minutes. When the CEL was on, the car would run extremely rich and dump fuel like crazy. I think he said he would see anywhere from 325-350 miles to a tank with conservative driving, and he was seeing something like 200-250 miles to the tank when the CEL was on.

9. You are truly running rich/lean and the ECU is not happy, thus the code is legitimate
There have been a few cases, noted below, when doing some type of tuning either via UTEC or a tuner with ECUTEK that results in the parameters being off scale and thus throwing the P0171. Perhaps you are truly running a bit too lean for the ECU's tastes and either the parameters for detecting the P0171 are not wide enough or your tuning is off. In the case of ECUTEK, it seems some people with DeltaDash can make some small adjustments in some cases, other cases might result in needing a professional retune (UTEC) or reflash (ECUTEK).

10. Dirty Idle Air Control Valve (IACV) - clean it out with carb cleaner or equivalent, however be sure to do this when the engine is cold so as to not cause deformation of the gasket. If you do happen to cause it to deform, a new one is about $8 from a Subaru dealership. Thanks samneric!

This is not meant to be an all-inclusive, be-all-end-all guide to the P0171, but it should be a good help for those experiencing it and banging their heads against the wall. Obviously, use your wits about you. If you have a CEL and get the code pulled, and you have 6 other codes thrown along with the P0171, you have OTHER problems that need serious attention. This is meant more for the people who randomly get a P0171 and can't figure out what they should do. PAWRX went through a lot of hell, money and effort to troubleshoot his car, and I have to be honest and say that I never thought the o2 sensor could cause something like this. Upon my research of the code, it appeared that #1-#6 were the most common reasons for getting it, but we both felt it would be worthwhile to post our experience in this matter for the good of the community. Again, any additions/corrections, please let me know and I'll edit anything ASAP.

Thanks - CirrusWRX & PAWRX

Last edited by CirrusWRX; 12-22-2004 at 01:28 PM.
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Old 12-29-2003, 09:16 AM   #2
Hank3
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Great write up!!!
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Old 12-29-2003, 09:24 AM   #3
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Here is a link to another thread about the P0171 code and the frount mount. If you go about half way down to where Phil from turboxs starts in you will see how you can throw this code as well


http://forums.nasioc.com/forums/show...hreadid=463660
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Old 12-29-2003, 12:57 PM   #4
PAWRX
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Just a little clarification. Excellent job Kyle, and thanks for swapping parts with me. You actually saved me a ton of money from what the dealer would have charged me in diagnostics fees. To clarify, my car actually ran fine. It wouldn't buck or stall or anything like that. It just went through gas. I was getting 200 miles per tank. That's why I really didn't think it could be the O2 sensor. All the threads i've seen in regards to O2 sensors going bad, the car would barely run. But mine ran fine. So if there is anyone out there racking thier brains over this. Do not overlook the front O2 sensor.

Ian
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Old 11-27-2011, 12:41 PM   #5
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so my question is my car is running lean wasting to much gas were is the first thing i do?

Ian[/quote]
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Old 12-05-2011, 01:05 PM   #6
markman
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Quote:
Originally Posted by HPREX View Post
so my question is my car is running lean wasting to much gas were is the first thing i do?

Ian


not sure if serious

Lean doesn't mean your car is wasting gas, but quite the opposite- it means the air/fuel mixture doesn't contain ENOUGH fuel, or at least your ECU thinks that it doesn't contain enough fuel. If it were running RICH it would be wasting gas.

Running lean is far more dangerous than running rich. You can seriously jack up your engine running lean.

The first thing to do would be to read the first post in this thread and start troubleshooting from there. Start cheap- check for leaks, clean your MAF, etc. Or take the car to a mechanic.
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Old 12-29-2003, 01:15 PM   #7
vile
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i'm shock about the suby service manuel, as they do not even mention the possibility of the front o2 being a culprit

they're trouble shooting steps are for coolant temp sensor, iat sensor, and maf.......

i would've thought for sure, you would include the front o2, as that is what is reporting the a/f ratio
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Old 12-30-2003, 04:26 AM   #8
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vile- knowing what I know now I would agree. But probably 99% of the time when the O2 sensor goes bad, the car will start running really rough and crappy. Not the case here.
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Old 06-01-2009, 11:40 AM   #9
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Quote:
Originally Posted by PAWRX View Post
vile- knowing what I know now I would agree. But probably 99% of the time when the O2 sensor goes bad, the car will start running really rough and crappy. Not the case here.
Mine ran OK with a bad front O2 sensor too, but I run open loop a lot (utec), when not just cruising.
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Old 12-30-2003, 11:44 AM   #10
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Any ideas on my situation? I have a P0171 that has come on ever since my fuel pump, turbo, injector, ground mod install. I have swapped the injectors, and it is still there. It comes on after about a week or two and stays on until I clear it. The car runs fine; I have not had any running issues or anything. It tends to also come on while idling at a stoplight.
I have a FMIC and a perrin short ram, and the clamps have been checked on those pieces.
Any thoughts that jump at you?
Thanks.
Mike
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Old 01-07-2008, 10:57 PM   #11
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Hi. I had a similar problem. It was the intake. My bet would be that the MAF calibration is at the border line with the new short ram. With mine I noticed that as soon as the weather cooled down it stopped happening. In the warm weather it had the same symtoms. Mine was a custom calibration that over time the ecu learned from the O2 sensor to a point that every now and then it was out of the O2 sensors range of keeping the ecu happy. Hope this helps!
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Old 12-30-2003, 03:00 PM   #12
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The only advise I can give is to go to the top of this thread and and start going down the list that CirrusWRX posted up. That's kinda why he posted it. to get all of the causes for this CEL into 1 thread. With my car it didn't take long to get the CEL. Just 2 driving cycles. Never got it at idle though. From your mods list I would say it could be any number of things - all of which were mentioned above. If you happen upon a new cause not listed here. Please add it.
Ian
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Old 12-30-2003, 03:05 PM   #13
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Sticky this


Full_Clip bumps for mod notice.
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Old 03-16-2004, 02:03 PM   #14
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wow... great post you guys... I had the code a little before I was going to put my car back to stock from stage 4 for the ref. I figured it probly wouldn't come back and 42 miles later normal cruising and the sucker popped up. Car runs fine. Slow by comparison of coarse

I will go down the list and post results. sounds like vacume lines and injector cleaner is a solid fist step. Thank you very much for all this info... There's so many possibilities I had no idea where to start.

Ash
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Old 02-01-2011, 01:29 PM   #15
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Quote:
Originally Posted by UURRXX_Ash View Post
wow... great post you guys... I had the code a little before I was going to put my car back to stock from stage 4 for the ref. I figured it probly wouldn't come back and 42 miles later normal cruising and the sucker popped up. Car runs fine. Slow by comparison of coarse

I will go down the list and post results. sounds like vacume lines and injector cleaner is a solid fist step. Thank you very much for all this info... There's so many possibilities I had no idea where to start.

Ash


Start with the easy stuff first.
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Old 02-01-2011, 02:12 PM   #16
Darryl
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Default that's an odd code to nail down

StreetKa
I would not be too interested in buying that car from what you say about it. I think you are rightfully hesitant: "mostly stock, rebuilt with Manely pistons". But that's just me.
As for the p0171, it's an odd one to track down. I had my front O2 sensor replaced and that didn't fix it. I cleaned the MAF and that didn't fix it. I checked for bad vacuum hoses, and found none. So I spent $50.00 and bought a cheap OBDII reader and just started clearing the code. Odd thing is the code would stay off for weeks then pop up again. I'd clear it, and wait for the next one to light off. Then it got to where I'd have to reset it like once a week. But now it's been six months since I've seen it. Go figure. It never did effect the drive-ability of the car; no change in startup, idling, power or fuel economy. Stranger than fiction.
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Old 02-01-2011, 04:48 PM   #17
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Quote:
Originally Posted by Darryl
StreetKa
I would not be too interested in buying that car from what you say about it. I think you are rightfully hesitant: "mostly stock, rebuilt with Manely pistons". But that's just me.
As for the p0171, it's an odd one to track down. I had my front O2 sensor replaced and that didn't fix it. I cleaned the MAF and that didn't fix it. I checked for bad vacuum hoses, and found none. So I spent $50.00 and bought a cheap OBDII reader and just started clearing the code. Odd thing is the code would stay off for weeks then pop up again. I'd clear it, and wait for the next one to light off. Then it got to where I'd have to reset it like once a week. But now it's been six months since I've seen it. Go figure. It never did effect the drive-ability of the car; no change in startup, idling, power or fuel economy. Stranger than fiction.
darryl
Yeah but for the record car felt weak. If that doesnt do anything to performance then its something else that makes a car run weird. So u can run like that without worries if happen? Right ? Just get o2 wideband and control afr to be sure i think will be best way to have piece of mind. Thanks anyway im not gonna buy it. I'd rather save up and buy newer 06-07 wrx wagon stock.
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Old 02-01-2011, 07:12 PM   #18
Darryl
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Default good call, StreetKa

Buy yourself some peace of mind (in a better Subie). Yes, no issues with my car because of that code. I always wanted to be sure I knew the CE light was not a (more) serious code. That's why I bought the reader. My old Bug-eye still runs like a young pup.
Good luck with whichever car you get.
darryl
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Old 09-27-2004, 03:52 PM   #19
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This is an excellent write up, thank you very much. I have a p0171 right now and took it to subaru and they told me it was because of my aftermarket cat-back. I didnt believe them seeing how ive had it on for about a year with no problems. I put my stock exhaust back on and the same code popped back up. I called them and there next excuse was it is because i have silencer removed, could be true i dont know. But upon reading this thread and checking the parts i feel i can without causing more damage i found that THEY not I, when doing my 30k overhaul just placed my stock airbox back on and ditn bother to slide to two tabs in the back under. So now i have the battery unhooked cause i was told that will clear the cel and i hope it doesnt pop back on. But is it possible that my cat back can cause my front o2 sensor to go bad?? thanks again for this write up, matt
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Old 09-28-2004, 09:18 AM   #20
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Quote:
Originally Posted by wrx_snobordr
THEY not I, when doing my 30k overhaul just placed my stock airbox back on and ditn bother to slide to two tabs in the back under.
Nope - this should be it. End of story

Smack the "service manager" upside the head next time you see him/her! And I agree with PAWRX - never heard of a catback causing a CEL, nor can I imagine it causing a front o2 sensor to go bad- the only thing I can think of is if you shoved banana's or melon's in there to block it off, but that's just retarded. Otherwise, you can run without a catback if you want to... of course, the floor might get a little warm... and the car might be a little loud... but doing any of that shouldn't cause your front o2 sensor to do anything differently.

Sounds like you're right on the money with disconnecting the battery for a little bit. That should clear your code, and you'll know within a few driving cycles whether or not you "fixed" (their) your problem!
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Old 09-28-2004, 04:27 AM   #21
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I have NEVER seen a catback cause a CEL.
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Old 09-28-2004, 07:55 PM   #22
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So its been 24 hours, some city driving, 2 highway trips and a short trip up to best buy. No CEL light and i appear to get better gas milage. When i had the light it was sucking up a quarter tank on one trip up and back to the city, roughly 50 miles. Thanks again for those who wrote the thread, and all others who put in info.

matt
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Old 11-01-2004, 08:08 PM   #23
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I first got this CEL shortly after a trip to the dyno. During the dyno runs they hooked up the WB O2 sensor in the front stock O2 sensors location. Is it possible that I am getting this code due to a leak? I have tried resetting the ECU, deleting the code, etc... but it always comes back, usually at idle. Any thoughts on this?
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Old 11-02-2004, 08:50 AM   #24
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Quote:
Originally Posted by tjamz
I first got this CEL shortly after a trip to the dyno. During the dyno runs they hooked up the WB O2 sensor in the front stock O2 sensors location. Is it possible that I am getting this code due to a leak? I have tried resetting the ECU, deleting the code, etc... but it always comes back, usually at idle. Any thoughts on this?
Was ANYTHING else done on the dyno AT ALL? When it comes back, is it at idle right after you turn the car on? Do you drive for a bit and THEN does it come on? Give a little more detail. Likewise, who tuned your car? Is there a chance your car is running way too lean and the code is truly telling you something that's "accurate"?

It's also possible the uninstall/reinstall damaged the o2 sensor. LOTS of possibilities, but you are on the right track focusing FIRST on "OK - what changed most recently?" And go from there. Did they mess with your airbox when on the dyno? Possibly bumped something or a clip? etc...
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Old 11-02-2004, 12:50 PM   #25
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Quote:
Originally Posted by CirrusWRX
Was ANYTHING else done on the dyno AT ALL? When it comes back, is it at idle right after you turn the car on? Do you drive for a bit and THEN does it come on? Give a little more detail. Likewise, who tuned your car? Is there a chance your car is running way too lean and the code is truly telling you something that's "accurate"?

It's also possible the uninstall/reinstall damaged the o2 sensor. LOTS of possibilities, but you are on the right track focusing FIRST on "OK - what changed most recently?" And go from there. Did they mess with your airbox when on the dyno? Possibly bumped something or a clip? etc...
I was tuned at the dyno, that is it, no parts added/removed. AFR @ idle is 14.5 to 15.0 according to my UTEC. If I let the car sit for 3 minutes idling it will come on, if I drive it I can get a few miles on before it comes on. I will give my full list of mods:

UTEC
TXS BOV
APS DR650
APS CAI
Walbro Fuel Pump
Perrin Up-pipe
PDE Downpipe
3" Catback to Apexi N1
Modded injectors
Grounding Mod
FP 18G
Blitz SBC-ID Boost Controller


I am going to pop out the O2 sensor today and check it out. Also, I'm going to try cleaning the MAF
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