The luxury of looking at the differential pressure sensor as your prime suspect under these circumstances is that you can diagnose its operation (or lack thereof) using a digital volt meter. I came to this website and found a link to another site (
Free-Auto-Repair-Advice: Understanding the Ford DPFE and EGR System) that provided a very simple, step-by-step procedure that took all of five minutes that provided the answer.
The good news was I had finally properly diagnosed the problem and could replace the defective part in two minutes; the bad news was that I had already dropped nearly $70 on an EGR valve that didn't require replacing. I had taken the EGR valve out, inspected its operation by applying vacuum and watching the pintle rod move as it should; I tested how well the pintle seat was, well, seating, and its integrity appeared fine.
However, instead of trusting my instincts, I took the word of a parts counter salesman who testified to selling lots of EGR valves. However, he probably never considered that lots of my predecessors there were making the same mistake I had, and his "wisdom" was borne of others' mistakes.
