PO 171 and 174 Error codes

I have a 2003 325i. The service engine light has come on intermittently, stays on for a couple of days, then goes out. The error codes are PO 171 and 174, banks 1 and 2, too lean. I had a mechanic check for vacuum leaks and he replaced one hose. The light went out for a few days and is back on again.

What's next before I go right to the replacing the sensors?

Hi. Faulty MAF. Jim

My list of possible causes of lean codes for both banks (in no particular order)….....

Vacuum leak:
- cracks in the accordion folds of the intake boots
- cracked or broken oil separator hose (there are 4 of them) (oil separator is also called crankcase vent valve, pressure regulating valve, PCV, etc)
- bad oil separator (labor intensive to replace)
- leak around the DISA valve gasket

Too little fuel:
- clogged fuel filter
- failing fuel pump

Bad MAF sensor: check the items above before replacing this expensive part, or at least try cleaning it first with CRC MAF cleaner. The O2 sensors would be the last thing to check in MHO, since they both would have to go out simultaneously for you to get both banks showing lean codes.

When I had lean codes for both banks, it was a vacuum leak caused by the commonly broken lower oil separator hose (the one that goes from the oil separator to the dipstick tube). Many people miss this when checking hoses, because it's a bit hidden unless you know where to look. Luckily, it is easy to check and easy (and cheap) to replace – just a pinch clip on one end of the hose and a friction fit on the other. Replacing it cured my lean codes. Here is a link with pictures on where to find it:

http://forum.e46fanatics.com/showthread.php?t=773551

Some lean codes that may be caused by this broken hose:
P0171, P0174 (on M54 and M56 engines)
P0170, P0173, P1188, P1189 (on M52 engines)

Originally Posted by jcjdavis1

Hi. Faulty MAF. Jim

My thought too. But I would recommend following gsbmw's advice before replacing the MAF as it is pricey.

Originally Posted by jcjdavis1

Hi. Faulty MAF. Jim

Nothing points to this as a cause. The MAF has its own codes #1, and #2, they are usually in a different family than the 0171 and 0174 codes.

OP, did your guy replace BOTH intake boots that lead to the throttle body? Many times the small diversion tube that sends air to the ICV will have small tears and cause the codes you have. I seriously doubt it is the MAF.

Originally Posted by smolck

Nothing points to this as a cause. The MAF has its own codes #1, and #2, they are usually in a different family than the 0171 and 0174 codes.

My X5 had codes P0170 and P0173 and the problem ended up being the MAF. Even the symptoms were the same (intermittent illumination of the SEL). I replaced the MAF and the problem was corrected.

Originally Posted by smolck

OP, did your guy replace BOTH intake boots that lead to the throttle body? Many times the small diversion tube that sends air to the ICV will have small tears and cause the codes you have. I seriously doubt it is the MAF.

I disagree. I highly suspect it is the MAF (but it wouldn't hurt to check the items gsbwm recommended before replacing the MAF).

Originally Posted by sunny5280

My X5 had codes P0170 and P0173 and the problem ended up being the MAF. Even the symptoms were the same (intermittent illumination of the SEL). I replaced the MAF and the problem was corrected.


I disagree. I highly suspect it is the MAF (but it wouldn't hurt to check the items gsbwm recommended before replacing the MAF).

So one car makes a generalization? Do you know how many cars I have fixed with those codes? I'll help, more than 10. Do you know how many were the MAF? I'll help again, 0. I never said it wasn't the MAF, but there are a bunch of other things it could be (and usually is) before we just make blanket statements like "it's the MAF cause mine died on me"

Originally Posted by smolck

So one car makes a generalization? Do you know how many cars I have fixed with those codes? I'll help, more than 10. Do you know how many were the MAF? I'll help again, 0. I never said it wasn't the MAF, but there are a bunch of other things it could be (and usually is) before we just make blanket statements like "it's the MAF cause mine died on me"

I could be wrong. Which is why I recommend he follow the checks gsbmw gave before replacing the MAF.

With that said given the codes and the symptoms I am betting on the MAF. I had almost the exact symptoms and codes which were resolved by replacing the MAF. And no, my one vehicle does not form the basis of my conclusion. I researched this problem extensively online before reaching my conclusion. Until I experienced this problem first hand I had no idea what a MAF was and how it worked. Through weeks of research it became clear to me the MAF was the likely cause of the problem. Once I was 99% sure it was the MAF I replaced it and the problem went away.

My money is on the MAF.

Originally Posted by Scott ZHP

Nope. I don't care. I am out on this thread. Best of luck.

Originally Posted by jcjdavis1

Hi. Faulty MAF. Jim

Hi. My original post was meant to be a question, forgot to put a ? in, not a statement. Jim

I'm having exact same problem. Smock said intake boots could be bad. Could anyone give me the part number for the boot? 2001 330xi

Originally Posted by KenN

I'm having exact same problem. Smock said intake boots could be bad. Could anyone give me the part number for the boot? 2001 330xi

Check #6 in fuel prep 13541438759
http://www.penskeparts.com/DiagramsM...6&rnd=06222010

Originally Posted by KenN

I'm having exact same problem. Smock said intake boots could be bad. Could anyone give me the part number for the boot? 2001 330xi

Did the OP ever resolve the problem and report back what it was?

Found the parts here: http://realoem.com/bmw/showparts.do?...05&hg=13&fg=15

He didn't tell me what's his fix was. Parts that Smolck referred are relatively cheap and doable (diy). I will try this. Thanks guys!

Like everyone else has said besides sunny, check the MAF last. I had the same problem and mine was fixed after replaced the small elblow hose on the lower intake boot going to the ICV so just make sure you check all hoses very thoroughly..

Originally Posted by BooMeR87

Like everyone else has said besides sunny, check the MAF last. I had the same problem and mine was fixed after replaced the small elblow hose on the lower intake boot going to the ICV so just make sure you check all hoses very thoroughly..

Um...here's what I wrote:

"My thought too. But I would recommend following gsbmw's advice before replacing the MAF as it is pricey."

Maybe your definition of last is different than mine? With that said, unless he is experiencing hesitation/stalling/rough idle, I would be surprised to see it was the air intake boot.

unless he is experiencing hesitation/stalling/rough idle, I would be surprised to see it was the air intake boot.

I had these same codes on my '01 330Ci, was running/driving perfect all the while...after reading several posts on Bimmerfest, took a look and found huge tears on upper intake boot. Taped up with silicone tape, and light went out. I am replacing both boots this weekend (will clean MAF while Im at it), light stayed off with taped boot all week...point is can have tears in boot(s) without hesitation and rough running.

Of course, this is just my experience...