The MAF measures the density of the air. Depending on how dense the air is the system either reduces or adds fuel to compensate.

There are both short and long term values. Short term fluctuations occur in essentially real time. Long term is more an average over time. If long term exceeds 20% then the SEL is illuminated.

I understand that but to one bank and lean? What you said would most likely to create a rich condition, isn't it? Not debating but learning through logic.

I'm not a mechanic so I can't explain why it would read for only one bank. I do know that my 4.4i had a fuel trim too lean bank one code and replacing the MAF solved the problem. As the part was so expensive, $350, I did a lot of research and the MAF was at the top of the list of causes (behind the fuel cap).

As for causing it to it being a lean/rich condition that would depend on how it's failing. Readings from the MAF are used to richen or lean out the fuel mixture depending on the air density (or what it thinks is the air density). A MAF works by heating a thin element to a high temperature. As air passes over the element it tends to cool it down. The amount of current used to maintain the reference temperature determines how dense the air is. Denser air cools the element more thus requiring more current. Less dense air requires less current. Thus the fuel mixture can either be leaned out or made richer.

I an not sure about the BMW-specific MAF, but many MAF sensors determine the air flow by heating a small wire or thin metal placed in the airstream. It then measures how much current is required to keep the wire at a constant temperature, e.g., the greater the mass of air flowing past the wire, the more current it takes to keep the wire hot. If it gets dirty, it insulates the MAF wire and the MAF then indicates a smaller mass of air flow going by the MAF than the actual amount of air mass going by the MAF. The computer then injects too little fuel, since the MAF has indicated less air was flowing than the true value. This results in too much air for the fuel injected and hence, "Too Lean."

As to why only one bank, well, in theory you should get an indication for both banks. But since an MAF problem due it it being dirty is typically a very slow process, gradually getting worse as time/miles pass, it is not unusual for the O2 sensor for one bank or the other to be the first to cross the threshold and indicate a "too lean" problem, e.g., one O2 sensor may simply be slightly different to the other O2 sensor due to manufacturing tolerances or aging, deposits, etc. So one of the O2 sensors will be the first to "pick it up," and the other will follow at some point down the road as the condition worsens.

Of course, it might also not be the MAF, and could be something specific to one bank or the other.

Thank you both. I see now. So if the maf measures less air going in it will send the signal to send less fuel.

I am still baffled by the one bank though.

That's how it is suppose to work. THe MAF also often has a temperature sensor in the assembly, and the engine computer get the air temperature and mass flow data from the MAF assembly. These two data points allows the computer to determine how much fuel to inject. The Pre-cat O2 sensors then measure the O2 content of the exhaust gases and, if it is too high, indicates a "too lean" condition. If it is only a little bit "too lean," the computer will use the O2 data to adjust the fuel amount, adjusting a parameter called "fuel trim." But if the lean condition it too great to correct with a "fuel trim" adjustment, it will set the "Too Lean" OBD-II code and turn on the SES light.

P.S. The post-cat O2 sensors purpose is simply to make sure the cat is working to ensure emissions compliance. The computer compares the O2 readings for the pre-cat and post-cat O2 sensors and uses this data to determine if the cat is really doing anything, i.e., defective post-cat O2 sensors will not impact engine operation or performance.

Hope that helps.

Great Tip! I'll check that out straight away.

Unfortunately I don't know of any way to test the MAF. The only way to know for sure is to replace it. And at $350 each it's an expensive mis-diagnosis if it ends up not being the problem.

I do want to say the code description you provided is not a fuel trim code so keep in mind we're not talking the exact same symptoms.

+1 on the intake gasket. I was getting the same error last year.