I suggest that you do a "google" search for "DPFE". There is a post listed from the Focus forum that has a great description with pics in regards to this sensor and it's operation.
It's not always the item that the code reader seems to pinpoint that is the real problem, It is often something else in the system that causes the item pinpointed to malfunction, which in turn causes us to start by replacing the item the code reader specifies and then we eventually find the base cause of the problem. We have to remember that code readers are only tools to aid in the diagnosis and repair of a problem.