Hi,
I could use some help with a code P0171 "system lean bank 1" on wife's '01 Pathy with 80,000 mi. Fuel-Premium I'm trying to avoid the black hole (in my wallet) of parts swapping as so many of my colleagues have unfortunately entered before me, and from the wide spectrum of fixes leading to a successful outcomes i.e. zero code, this code fits no clearly defined set of causes.That said, Here's what I'm working from:

Vehicle idles/runs excellent except for some detonation under moderate load. No hill = no pinging. Checked for air leaks and found none. Cleaned MAF despite the logic that says BOTH banks should set codes but heard from Nissan tech claiming to have seen these go bad and only set a code on one side-anything to confuse me. Immediately upon start up, the code for opposite side-0174 "too lean Bank 2"- set, took a 35 mile trip with wife on interstate with plenty of uphill grades and voila-zero codes. An 8 mile trip next morning on flat hwy doing 65 and the 0171code returned. Hmmm.
Fuel trims over three cycles are as follows:
Short term fuel trim Bank 1 was unchanged over three cycles at 1.6%. Long term Bank 1 also unchanged but @ 32.8%. Short term Bank 2 averaged (-) 2.6%, and Long term Bank 2 averaged 31.5%. However, following the MAF cleaning and the temporary switching of the codes (171 to 174 then back), and the "clean" 35 mile drive, the trims changed substantially: Short term Bank 1 went to 6.3%, Long term Bank 1 went to 31.3%, Short term Bank 2 went way up to 3.9%, and Long term Bank 2 dropped to 28.9%.

Could the variance in trim indicate a bad MAF? One other thing I forgot to mention is the presence of "rotten egg" odor that always accompanies the detonation when climbing. My thinking at this point---Since the pinging is a lean condition and in itself would not cause the sulphur odor, could the MAF malfunction to the point of creating a "lean" condition which in turn is causing the pinging and then try to adjust for this condition by dumping excessive fuel back into the cylinders and as such causing the rotten egg smell of an over-rich condition? Bad MAF? Seems like a reach but...
This one's working my mind overtime. I'd be grateful for any help out there. Thanks.

Where are you located? Are you at any type of altitude?

Sea level.

Good news. A second cleaning of the MAF, running a can of injector cleaner and a refill with premium vs regular previously used (by me , not the wife -Wife-1 Mechanic-0) & the code stayed off long enough to zoom into the test only station, and PASS--Big. That, was, my MAIN concern. Spell RELIEF.

I'm entering a new phase of auto diagnostics with the sensor dependent engine systems, and getting input via the internet has been, and will continue to be my first course of action-fight fire with fire so to speak. And in the case of this code 0171 there's so much ambiguity surrounding it and no doubt other codes too, that for those who are passionate (sensible) about confirming a problem before throwing solutions at it, foregoing the use of forums as a critical diagnostic component, in IMHO is nothing short of what parts changers do-gamble at the owners expense.
Thank you for these forums. They are worthy of support.
That said, this was one of those experiences I'm glad to put behind me, when I really couldn't see the end in sight. The whole thing cost around $70.. bucks and probably 5 hours of diagnostic and checking codes to see changes over a few days driving. Therefore, I am not complaining. I did locate a MAF for a Maxima for around &100. buck which I would have "tried", if the problems had not been "solved".

A homily.
Perhaps one day the automotive industry will take the next step in computer control and create fail-safes that incorporate sensors with enough range and sensitivity to accurately discriminate and/or cross check inputs from every input in relationship to one another in the particular system being monitored-air/fuel, ignition, exhaust, emissions,etc.--inputs that encompass a much narrower and more clearly defined set of parameters which does not allow for variance making the potential for setting highly ambiguous codes much less likely than at present.
Presently , what appears to be the industries answer to engine control is single sensors measuring for a predetermined response at a source, for example MAF, in-putting predetermined values to be read by the ecm as being either acceptable/unacceptable and, when out of value, setting a code. The problem is, we all know that the properties and physics surrounding combustion, heat displacement, loads, etc....create the potential for far more changes and less predictability not the least of which manifest themselves as intermittent occurrences, than are ever accounted for by engine control sensors.

What now is a much less "closed" loop in engine control then the auto industry may to admit to , may work a well a portion of the time, but in the case of diagnostics, it is barely off the ground, imho, and is in need of some major improvements. When I was taking on this code 0171, I felt like I'd been given a map without a compass.

BTW what is your assessment of the fuel trim values I listed? Be interested to hear. Thanks, for the reply. rm