# Fault Code - P0171 - Bank 1 too lean



## bmw_akw (Feb 4, 2017)

Can anyone help me with this one? The car seems to be running fine other than the EML on the dash and a slight fluctuation in revs when stationary.

I'm thinking I should look to change the Diverter Valve first?

The car is standard other than a Blow Off Valve (which I'll be removing), RamAir intake and Milltek Cat Back - all of which have been on the car for a fair few thousand miles.

Any help would be really appreciated.


----------



## barry_m2 (Jun 29, 2015)

Bank 1 too lean is probably referring to the lambda sensor in that bank not giving a correct reading. Better to plug it in to VCDS and get a reading off that, it will be more descriptive.


----------



## bmw_akw (Feb 4, 2017)

Oh ok... I didnt realise VCDS would give more info.


----------



## FNChaos (Nov 30, 2016)

A Bank1 lean condition means either not enough fuel or too much air is being detected. Much more likely you have a vacuum leak rather than a fuel issue. Check for damaged vacuum & PVC hoses.
If there is nothing obvious, you will need to monitor your short-term & long-term fuel trim (STFT, LTFT) to deduce what is going on.


----------



## bmw_akw (Feb 4, 2017)

Good advice, thank you. MPG has dropped from 26ish mpg to 22mpg.

And the exhaust seems to be popping a little more on overrun (which does sound nice  )


----------



## xpanel (Jun 16, 2013)

I had the same issue. Bank 1 too rich, Bank 2 too lean. I was able to fix the bank 2 by resetting it via vcds, but i cant figure out how to correct bank 1 too rich, been driving about 2k miles with check engine.

I checked for vaccum leaks, checked MAF, checked all 4 O2 sensors, Still cant fix the issue.


----------



## sunkiss (Jun 26, 2012)

Had the fault code of Bank 1 too lean due to a leaked vacuum, replaced to fix the issue.


----------



## xpanel (Jun 16, 2013)

grats, at least you figured it out


----------



## SLine_Tom (Oct 20, 2017)

I have now developed the Bank 1 too lean code, going to take it to the garage at the weekend, it doesn't sound to pricey, but was wondering if anyone had any ideas of worse case scenario ?


----------



## bmw_akw (Feb 4, 2017)

I endeded up:
- removing the blow off valve (and throwing straight in the bin)
- replacing the spark plugs
- replacing diverter valve
- replacing the PVC valve (which I believe rectified the issue).

Since then the average mpg has increased from 27mpg to a whopping 31.7mpg

I might just add, I replaced the above as the car had done almost 70k miles and as they were all pretty cheap parts, figured I may as well do them all now and save myself the job later down the line.

I fitted all the parts myself with ease and I'm certainly no mechanic!!


----------

