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Q: Vince on 2004 Mercury Monterey

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The fault that is coming up is lean bank 1 and 2 fault. I have had the fuel filter and the drive shaft positioning sensor replaced. When driving the van at times the RPM will fall below 100 and it idols real rough.
(2) Answers
I am not Vince but I have repaired many 'lean bank 1 and 2' codes i.e P0171 and P0174. Here is a lean fuel trim code diagnostic article that I wrote for RepairPal that goes over a step by step inspection procedure for these codes.

http://repairpal.com/OBD-II-Code-P0171-and-P0174 many people have had success with these inspections --> good luck !
You might want to check for a vacuum leak. I had the same codes on mine and after changing the O2 sensor the code changed to another sensor. that is when I noticed the PCV hose had a nickel size hole in it at the rubber bend.