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Old 04-28-2009, 09:46 AM   #1
Heider
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Default CEL P0171 - bad MAF or O2 sensor....or something else?

This past weekend I got a check engine light, so I looked up the code with my accessport and it listed it is "P0171 - System too lean (bank 1)".

I searched the forums and most people with the same issue said it was probably a bad O2 sensor or maybe a dirty MAF. I don't want to just drop $200 on a new O2 sensor and find out the problem has not gone away, so hopefully I can get a more solid suggestion...

At the time I was running stage 1 93 octane map, and I noticed that at idle the RPM's would drop from around 500 to 100 a few times and back up to 500. The engine also felt like it was struggling a bit. This would only last for a few seconds at a time and then stay solid at 500rpm. I also felt a slight power loss when on the throttle.

So I put it back to the stock map, and the idle rpm's seems stable (still CEL). But I noticed that when idling the engine vibration will briefly increase and my boost gauge shows the pressure suddenly fluctuating from -7.25psi to -10.87psi.

Info:
2005 WRX
All stock except the stage 1 map I was running
~105,000 miles
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Old 04-28-2009, 10:01 AM   #2
Rodwrx23
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I would say that at 105K miles your O2 sensor is probably borked. If you don't want to swap it out just yet, you can try cleaning the MAF sensor and also cleaning the IACV and replacing the gasket. Follow this up with an ecu reset and see how the car responds.
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Old 04-28-2009, 10:23 AM   #3
Heider
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Quote:
Originally Posted by Rodwrx23 View Post
I would say that at 105K miles your O2 sensor is probably borked. If you don't want to swap it out just yet, you can try cleaning the MAF sensor and also cleaning the IACV and replacing the gasket. Follow this up with an ecu reset and see how the car responds.
Ah I didn't think about IACV, though after going back to stock map the idle seems stable. Any idea where to get replacement gaskets, or do I just have to ask the dealer?

I'll probably go with the idea of cleaning the MAF first since it will only cost me a few bucks for cleaner (and maybe a gasket).
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Old 04-28-2009, 10:41 AM   #4
KitchenerWRX
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Could be as simple as something like a vaccuum leak, or dirty MAF sensor
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Old 04-28-2009, 10:48 AM   #5
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system to lean.

this DTC is set when your fuel trim (positive in this case) goes above 25%. This means that the ECU is sending 25% more fuel at any given point than it would normally like to. This can be caused by vacuum leaks, clogged fuel filters, faulty pressure regulators, and kinked lines. Along with 1000 other things. MAF's and o2's could be the cause also. But there are cheaper ways than just replacing the part to find out.
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Old 04-28-2009, 10:50 AM   #6
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Originally Posted by turboL View Post
But there are cheaper ways than just replacing the part to find out.
can you elaborate? The dealer wants to charge me $100 just to run it on the diagnostic.
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Old 04-28-2009, 11:01 AM   #7
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i got the same code, turned out that my intake cone was f---ed so i got a new one and it runs awesome now. it can be numerous things, o2 , maf, filter, emmission lines, boost seleniod lines. leak in the system. take your pick go from the least expensive to the next.
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Old 04-28-2009, 11:08 AM   #8
Rodwrx23
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When I fixed mine I cleaned the filter, IACV, MAF sensor, checked for leaks and found none so it's hard for me to say exactly what fixed the problem. Regardless, it wasn't a lot of work to do these things. You can get a new IACV gasket from the dealer (Part# 22659AA120). I had already ordered a new front O2 sensor from SGP for $150 so that went in as well after it the code went away. Figured why not and I sold my old unit for $70.
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Old 04-28-2009, 11:13 AM   #9
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Quote:
Originally Posted by Heider View Post
Ah I didn't think about IACV, though after going back to stock map the idle seems stable. Any idea where to get replacement gaskets, or do I just have to ask the dealer?
http://forums.nasioc.com/forums/showthread.php?t=781242
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Old 04-28-2009, 12:55 PM   #10
Rickyh
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You probably have a leak somewhere after the maf sensor. It is sucking in unmetered air. Look for a loose clamp or hose that came off. That is the cheapest and probably the most likely cause.
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Old 05-01-2009, 10:17 AM   #11
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I ended up cleaning the MAF sensor a couple days ago and resetting the CEL. The light has not come back on, but I think the problem is still there because my boost vacuum is fluctuating too much at idle. Gonna have to check all my vacuum lines. I ended up buying a new oxygen sensor anyway since my car is high mileage. They had a decent deal at subarugenuineparts.com. Will post up the results after I get the new sensor installed.
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Old 05-01-2009, 01:18 PM   #12
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DUDE check/change your oil and filter... sounds like exactly what happened to me before I blew a rodbearing and now need a new shortblock. I would say spray clean your maf... check filters/connections at intake if you have one, and then make sure all seals are good.

Seriously, check oil/filter NOW.


Also, autozone will pull codes for you for FREE. Head there, read the code and see what it comes out as and report to us. Good luck, hope you can fix it before it's really a problem.
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Old 05-12-2009, 09:23 AM   #13
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Issue finally resolved. Here are the steps I went through...

1. cleaned MAF
2. changed oil/filter - was already time to change anyway
3. replaced oxygen sensor
4. fixed tear in air intake filter
5. cleaned IACV

as I said above, I reset my CEL after cleaning the MAF and the light stayed off, but the boost pressure was unstable.

after I finally replaced the O2 sensor, the boost pressure stabilized, but I think fixing the tear in the air filter helped as well. thanks for everyone's help!
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Old 05-12-2009, 02:51 PM   #14
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Default ooh! ooh! me too!

Same code. Autozone cleared it out, and it returned two days later. P0171. My '02 REX has less than 57k miles on it, but I replaced the front O2 sensor 16 months ago. Anyway, I bought an OBDII reader, and cleared the code twice. Haven't seen it in two weeks now. Made me wonder if I got caught in the seasonal gas change-over. I haven't done anything, but check for vacuum leaks. The car idles dead solid on 750rpm, starts right up, runs with no hesitation, and drives strong. Gas milage is the same as always, 17mpg in he city on 93 octane. Go figure.
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Old 05-12-2009, 04:01 PM   #15
Heider
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Quote:
Originally Posted by Darryl View Post
Same code. Autozone cleared it out, and it returned two days later. P0171. My '02 REX has less than 57k miles on it, but I replaced the front O2 sensor 16 months ago. Anyway, I bought an OBDII reader, and cleared the code twice. Haven't seen it in two weeks now. Made me wonder if I got caught in the seasonal gas change-over. I haven't done anything, but check for vacuum leaks. The car idles dead solid on 750rpm, starts right up, runs with no hesitation, and drives strong. Gas milage is the same as always, 17mpg in he city on 93 octane. Go figure.
darryl
yeah not sure what would cause it to show up and stay off after clearing it (without making any other changes).

this thread was useful...

http://forums.nasioc.com/forums/showthread.php?t=475620
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Old 05-12-2009, 07:00 PM   #16
Darryl
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thanks for the link, Heider. useful stuff there. Since I did nothing to fix the cel, I expect it to return eventually, and will be surprised if it doesn't. All things considered, I'm betting it's the front O2 sensor. As I said earlier, the car runs just fine.
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Old 05-22-2009, 12:00 PM   #17
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Quote:
Originally Posted by Heider View Post
yeah not sure what would cause it to show up and stay off after clearing it (without making any other changes).

this thread was useful...

http://forums.nasioc.com/forums/showthread.php?t=475620
the ecu needs to see the car maintain a constant speed of around 50-55mph for about 4-6 miles for it to "reset" and re look for the codes. I am having the same issue right now and thats what i was told because my ecu would come up "system not ready" when they threw it on the emissions machine.
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Old 05-22-2009, 05:28 PM   #18
Darryl
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Quote:
Originally Posted by Darryl View Post
thanks for the link, Heider. useful stuff there. Since I did nothing to fix the cel, I expect it to return eventually, and will be surprised if it doesn't. All things considered, I'm betting it's the front O2 sensor. As I said earlier, the car runs just fine.
darryl
well here it is 5-22, and still no return of the ce light. How odd.
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