Vince on 2004 Mercury Monterey

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.

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. many people have had success with these inspections --> good luck !
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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.