NEVER I repeat NEVER let a parts guy diagnose your vehicle,all due respect to any parts people that read this but I have made ALOT of money fixing autozone lean code/o2 sensor diagnostics. Just get them to pull the code and ask us,we got you're back....but not you're o2 sensor,just kidding.
Does this year still have that vacuum outlet under the plenum? The one that is about 3/4 in diameter then reduces down to say 3/8 and turns into mush after a few years? Almost need a scope or mirror to see it and can be a pain to change. I know you know the one i am refering to, changed some on older Taurus with the DOHC engine, just not sure if this one still has it. Just curious.
3.0 DOHC Ford engines, which you have, are famous for vacuum leak @ large rubber (reducing tubing connector) under the intake manifold. Hard to see this! Also the entire pcv valve circuit, including the valve! Is idle speed erratic, up and down after running a few minutes?
They love to sell parts; Could be but need further diagnosis to be sure. Check for vacuum leak, air intake tubing on engine side of MAF sensor. Another thing I have had intake manifold leaks from loose bolts.