Check engine light and code P0171?

Hello, it sounds like you may have a vacuum leak, is there a high idle when stopped? Do you have a scan tool that can read live data? Here is a guide to help finding vacuum leaks, some are usually pretty loud. A hissing noise is what you would hear.
If you do have a scan tool that has live data, look to see if there are any odd readings, such as Long- and Short-Term fuel trim numbers at idle and at 2500rpm, if they improve at higher RPMs it's a vacuum leak. If no smoke machine is available, you can try spraying some water around the base of the intake manifold where the gasket is and see if you hear the water being sucked in due to the strong vacuum at idle. I will look up your vehicle and see if there are any technical service bulletins. It usually takes a lot for Fords to set codes, I own one too and it very seldom sets any codes. Weak fuel pressure can set a lean code, a stuck oxygen sensor can do the same. Read through the guide and I'll look up the fuel pressure spec.
Do you know if your vehicle has dual overhead camshafts or just one camshaft? Or you can give me your vin number and I can look it up from that.

If you do have a scan tool that reads the code, there should be a option with the code to look at Freezeframe Data, it can tell a lot about what was happening when the code set.
Also are you having any other symptoms such as lack of power, surging, hesitation?
There was a Recall on the Fuel delivery module, for a new design. This is way back from 2004, but shows they had a problem with the fuel system, and low fuel volume can cause a lean code.
There was also a Technical Service Bulletin on Vacuum leak detection, pages 3 to 7 below.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge