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Thread: P0171 & P0174 codes.......WTF??

  1. #1

    P0171 & P0174 codes.......WTF??

    OKay, I'm getting the usual P0171/P0174 codes. I pulled & inspected the intake boots & F connector, no cracks. Did a smoke test, didn't see anything. Sprayed the MAF sensor off with solvent too. The car is running fine but those codes keep coming back!!! Any suggestions for where to look next???

    Thanks in advance!

  2. #2
    Quote Originally Posted by
    A code P0171 may mean that one or more of the following has happened:
    The MAF (Mass Air Flow) Sensor is dirty or faulty Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
    There could be a vacuum leak downstream of the MAF sensor
    Possible cracked vacuum or PCV line/connection
    Faulty or stuck open PCV valve Failed or faulty oxygen sensor (bank 1, sensor 1)
    Sticking/plugged or failed fuel injector
    Low fuel pressure (possible plugged/dirty fuel filter!)
    Exhaust leak between engine and first oxygen sensor
    Since you did a smoke test and cleaned your MAF, we can rule out most of those.

    How many miles on the Oxygen sensors? How many miles on the Fuel Filter?

  3. #3
    I just bought the car a few weeks ago so I can't answer the mileage questions. I'll replace the fuel filter & PCV valve but I thought bad O2 sensors threw their own separate code??

  4. #4
    Join Date
    Oct 2016
    Atlanta/Macon, Georgia
    Check the PCV system. Its one of the PITA maintenance items that can cause code gremlins. Unless you have documentation that it was recently done, it probably needs replacing.
    Edit: I guess a failed PCV would probably fail a smoke test...
    Last edited by JPMo; 11-28-2020 at 10:03 AM. Reason: clarification

    2004 Titansilber Metallic BMW 330i ZHP

  5. #5
    Quote Originally Posted by JPMo View Post
    Check the PCV system. Its one of the PITA maintenance items that can cause code gremlins. Unless you have documentation that it was recently done, it probably needs replacing.
    Edit: I guess a failed PCV would probably fail a smoke test...
    A failed PCV would not fail a smoke test.

  6. #6
    Turned out there was a little hole in the oil return hose that goes from the bottom of the CCV/PCV down to the oil dipstick tube.
    I ended up replacing the CCV, intake gaskets and a pile of other parts.
    The P0171/P0174 codes cleared & stayed away for a short time but they're back again. I think I'll try the smoke test again. Maybe I'll get lucky and it'll just be a damaged the DISA gasket. I watched a bunch of youtube vids by 50sKid that really helped with taking everything apart. That guy seems to do a really nice job with his videos and explains things very well.

    Thanks for everyone's help!

  7. #7
    Join Date
    Jan 2013
    Central California
    I had the same problem about 6 months ago. I guess that hose is a common failure. I found the hose to be the cause by spraying carb cleaner on certain areas while monitoring short term fuel trims with the OBD Fusion app on my phone. As soon as I hit that hose with the carb cleaner the STFTs jumped toward zero. I knew I had found the cause. I think this method works better and is easier to do than a smoke test. I don't care if it gets me labeled a heretic.

  8. #8
    Lazy o2 sensors can also cause those codes. By the way, a failed MAF doesnt always throw a code either. If you are certain you have all the common leaks handled and have more than 100k miles, I’d replace the 02 sensors. Even if it isnt the source of your code, it needs to be done anyway. Then I’d start drilling down to the MAF.

    Check out my youtube channel for all kinds of E46 DIY's
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  9. #9
    Join Date
    Jan 2019
    Ft. Lauderdale, FL
    I believe these are the same codes I got when I first bought my car. Despite no leaks, new O2s, etc, I flashed the ECU with updated software that BMW released to fix this problem. Do you get studdering at certain points in the RPM band?
    '04 330i ZHP sedan | Mystic blue | Alcantara | 6MT | 111k
    '98 740il | Schwartz 2 | Sandbeige | 5AT | 238k | lowered on Bilsteins w/ B&G springs
    '00 540i sport | Titanium silver | Black | 5AT | 120k
    '85 Mustang GT convertible | Medium charcoal metallic | Gray | 5MT | 215k | one owner, all original

  10. #10
    No studdering at all, it runs smoothly. I haven't had time to replace the O2's yet but I think I'll put that on my list. I have 145k on the car & I have no idea if they have ever been done.

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