DIY - FIXED N52 Lean Engine Codes (P0171, P0174)

[trentonsandler]

Hey all, after dealing with lean engine codes in both banks in my 2010 128i for almost 2 years, I just wanted to share how I finally fixed them! I also wanted to give a step-by-step guide of what I would have done from the beginning to have it fixed in a week (hopefully save you the forum-scouring).

Everything I've learned about my car is self-taught, I've done several performance modifications and some interior/exterior work as well. But I am an expert by no means, so feel free to correct me on any of this!

Steps to fixing lean codes on an E82:

1) Buy a scanner tool and app. I used the VeePeak LE tool with the OBD Fusion app. With this you can check the car's short term fuel trims (and the codes), and figure out where to start. From what I understand, if your fuel trims start low/normal, and get worse as you accelerate, it is most likely a fuel delivery or sensor problem (this is what I had).

If your trims start high/bad and improve as you accelerate, you most likely have an air leak.

For an air leak:
- check for obvious leaks
- do a smoke test (you can take it to a dealership or DIY it)
- use carb cleaner to manually test (I didn't like this one)
Then simply replace the part that is leaking!

For fuel/sensor problems:
- start with the fuel, as it is pretty easy and can eliminate a lot if it's good
- tap into the fuel rail at the engine with a fuel pressure test kit (pretty cheap at harbor freight) and check that it is at ~72psi at start, constant even when giving it some gas, and doesn't bleed much within the fist 5 minutes after shutting off the car

Now for sensors:
- O2 and MAF are most common, but MAF is a common denominator between both banks so that is the most I'd say
- to check O2s: use your reader and make sure the values aren't erratic, and both banks follow along with each other. It is uncommon for BOTH to fail suddenly, you would most likely know.
- for MAF, check the readings at idle, and check freeze frame data. At idle, it should be somewhere in the 3-5g/s range I believe. It should go up significantly as you accelerate.
- a soft-failing MAF is common, where it just can't keep up at high speeds. This was the problem I had. Freeze frame data showed that the MAF was only reporting 8g/s going ~70mph--hence the lean code.

I replaced the MAF days ago, and no code since. A couple hundred miles both on highway and city and I'm pretty certain it's gone for good.

That is the extent that I know but hopefully that can give someone a better game plan, and less time digging through forums and never ending youtube videos. If any of this needs updating or correcting lmk, but I tried to keep it simple because there is a TON of information out there about this relatively basic problem.