If you looked at the old plugs when you pulled them out that would be the best way. I had a clogged one and it would make the plug on that cyclinder white. This also caused the PCM to really oscillate the fuel between rich and lean on that one bank as the O2 was trying to maintain it;s closed loop AF ratio.

 

Sounds like an O2 sensor providing the plugs were gapped correctly. Run a tank of 91 octane and then try a tank of 87 (or whatever regular is in your area), and check mileage again.

 

Wouldn't an O2 sensor show on the scanner? The mileage seems wacked, but I'm willing to try anything (within reason). Any sensor more likely than another & how much do they run?

 

Once you download the code and look it up, it'll give you a brief breakdown of what is wrong. I had a code around 70K on my 'Rus, and it was the right bank O2 sensor. A friend of mine took care of the work for me as I was busy with work and never did get the details.

02 sensors are around $50, but will get pricey if you get it at a Ford dealer.

 

2000 Duratec - 60,000 miles.
Bad gas mileage - fixed the usual suspects ( DPFE sensor, spark plugs, air cleaner, fuel filter). Home code scanner & dealers scanner show clean, car runs OK but gets like 17 mpg in town, maybe 20 on the highway.
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Okay, I gonna ask a touchy question: what are your driving habits like? A perfectly running Duractec might these numbers with a lead foot.

If someone has the specs on the O2 sensor you could just back probe it with a DVM. That might give you some idea they're getting lazy.

 

Get a tail pipe emissions test if you can. The test will show engine emissions performance. Better than R&R of multiple components. Cheaper too.