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OBD II codes P0171 & P01140

(2001 Nissan Pathfinder)
in Flagstaff, AZ on May 06, 2010
Service engine soon light on and OBD II codes P0171 & P1140. "Fuel system lean on bank 1" along with "mass or volume air flow circuit range/performance problem.
This 2001 Pathfinder has 135000 miles on it and the Fuel mileage and engine performance are very poor when compared with my other 2001 Pathfinder which has 172000 miles on it. After I replaced the fuel filter and the air filter the service engine soon light went off for a few days, then came back on with the same two codes stored. When accelerating power drops off even more as soon as it reaches between 3600 and 3800 RPM. I checked all vacuum related hoses for leaks except the PCV hose as I cannot locate it. All hoses are in good condition and still feel nice and plyable and none touching where they can wear through from vibration. Used carb/throttle cleaner to try and detect a leak, however have not checked the intake manifold itself. With these two codes stored together would the highest probability be the Mass Air FLow sensor? Note that the new air and fuel filter on this Pathfinder is from Autozone, whereas the air filter on my good running Pathfinder is from NAPA. I can check the MAF voltages, however I don't have a scanner to check the barometric pressure or long and short trims. Would the MAF be the highest probability considering the symptoms I have described?
Displaying 1 answers & 1 comments
Popular Answer
on May 06, 2010
Here is paper I wrote on these very tough codes

I do Emissions work in CA and this code really throws people. Yes it could be a Mass Air Flow, BUT be sure to check for any and all vacuum leaks. It is NOT an bad Oxygen Sensor, which so many people run out to change and are then are sorry.
on May 07, 2010
Thanks for your response. I learned a lot from the paper you wrote prior to my posting my question. It is really good information. Like you, I don't change parts simply on a hunch. I would either check/test the specs/operation myself or if it were beyond my capabilities/tools, have a mechanic test the specs/operation and if the component failed to meet the operational specs, then and only then would I replace it. It does save time and labor, however, if you focus on the most likely component first, then ,if it passes muster, move on to the next most likely component and check it's specs/operation. I will ensure that I have no air leaks, clean the MAF sensor, check the voltages and the ground to it and if no problem found I will turn it over to a mechanic with the right tools. Thanks.
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