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Discussion Starter #1 (Edited)
I have a 2004 E46 325CI with the M54 engine in it. A few weeks ago, my check engine light came on. I didn't think much of it as I was getting on the throttle pretty good getting onto the interstate (figured it was just a retarded O2 sensor due to my lead foot). Took it to the shop to get the tires rotated and codes read and cleared.

Ended up being just the P0171 (Bank 1 too lean) but it wouldn't clear even after they tried. I've been looking into getting a new O2 sensor, so I got the codes read one more time just to be certain. Now I have both P0171 and P0174 (Bank 1 & 2).

I've been reading around how a lot of people have associated this with their own vacuum leaks which they diagnosed. I'm going to take a look under the hood tomorrow for any leaks. I haven't noticed any sort of lag, loss of power, bad idle or anything. Car still runs great as far as I can see (maybe a little loss of MPG)

Is there any specific places to look? I'd hate to spend $350+ on new O2 sensors if that isn't the problem. What do you guys think?
 

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Check your intake boot... On my old zhp I had this issue and at that time my car was under warranty so I brought it to the dealership and they replaced the intake boot cause it has torn... I hear it's kinda a common problem


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Usually the last item to condemn are sensors. They don't fail as often as people think, and rushing to sensors as the culprit prematurely can really be a waste of money. P0171 and P0174 are notorious for vacuum leaks. If your CEL light is on and won't go off, you have some investigating to do with your eyes and hands. Intake books, CCV, vacuum lines, gasket failure on valve cover etc.

Also, if your CEL comes on at highway speeds, faulty MAF's have been a source of the problem as well.

I suggest you invest a little money into an OBDII reader that can support freeze frame and read LIVE data as well. This will help you.

Search e46fanatic jfoj and read the links in his signature. Should direct you where you need to go.

HTH
 

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Check your intake boots and the oil line from the ccv to sip stick. Im dealing with this codes right now myself
 

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Had the same problem with my old 01 330Ci and it turned out to be a faulty MAF (Mass Air Flow) sensor. Now I have an 03 330Ci with 65k miles on it and I just recently (a week ago) started getting "P0174" & P0171" CEL light again. Again it could be like the other mentioned as well. Usually caused by a vacuum leak somewhere but if all else fails take a look at the MAF sensor. Could be dirty or needs to be replaced.
 

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I have a 2004 E46 325CI with the M54 engine in it. A few weeks ago, my check engine light came on. I didn't think much of it as I was getting on the throttle pretty good getting onto the interstate (figured it was just a retarded O2 sensor due to my lead foot). Took it to the shop to get the tires rotated and codes read and cleared.

Ended up being just the P0171 (Bank 1 too lean) but it wouldn't clear even after they tried. I've been looking into getting a new O2 sensor, so I got the codes read one more time just to be certain. Now I have both P0171 and P0174 (Bank 1 & 2).

I've been reading around how a lot of people have associated this with their own vacuum leaks which they diagnosed. I'm going to take a look under the hood tomorrow for any leaks. I haven't noticed any sort of lag, loss of power, bad idle or anything. Car still runs great as far as I can see (maybe a little loss of MPG)

Is there any specific places to look? I'd hate to spend $350+ on new O2 sensors if that isn't the problem. What do you guys think?
I had holes in my intake boots. Actual missing bits. Anyhow, I had great performance and smooth idle. The only time I'd CEL was cruising at normal speeds on the highway after the car was nicely warmed up. My fuel mileage got progressively worse though, but I just blamed it on winter ice/snow removal idling. New boots on and I've gone from 20 mpg up to 24.5 mpg, no more CEL.
 

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Most likely vacuum leaks, but posting freeze frame data would help diagnose.
 

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Discussion Starter #9 (Edited)
You guys are awesome! :thumbsup:

Took apart my intake and found electrical tape wrapped around the corrugated rubber that connects the upper and lower intake boots :facepalm: The tape had rotted away a bit and when I tried to flex the pipe, you could see hairline holes all around it :tsk:

Also, the lower boot's connector had a nice gash in it. Not huge but enough to let light through so I'm off to BMW to grab some new boots and some MAF cleaner just to rule it out.

Once I find the problem, will the CEL go out automatically or do I have to take it to a shop and have them clear them each time I think I won?

 

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Discussion Starter #11
Well thanks to you guys, my baby is finally running smooth. My intake was cracked horribly! I cleaned out the throttle body and the idle control valve and she runs great now! MPG has gone up dramatically as well. The codes didn't clear on their own after maybe a 10-15 min drive, so I took it to a shop and had them clear it. Sure enough, CEL stayed off.

Thanks for all the help guys!
 
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