I can give you some information.
The sensor you are speaking of is the DPFE (or EGR pressure) sensor. This is a very high failure part on the Ford vehicles. They have a few different styles of this sensor, all do the same thing. This, however, is a factory recalled part. If you have been paying for replacement, you shouldn't have had to. The shop should have informed you of this recall. Here's a copy of an owner's letter:
This sensor will also not cause a specific code to be stored. I'm going to venture a guess and say that you have a P0401 code stored (EGR flow insufficient.) This is the code that we usually see on these cars. And yes, key on, engine off, you should see right around a 1 volt signal from this sensor. I would not recommend an aftermarket sensor on this, either. I've installed 4 bad aftermarket DPFE sensors on an Explorer in the past. Got a dealership sensor, and it was fine.
There are other things that can cause this code, too. If the EGR valve has failed, this code can be set. If the EGR passages are plugged, this code will set. The only way to check this is to remove the EGR valve itself, and look into the passages.
I would recommend asking if this was the sensor that was replaced, and if it was, ask why you were charged for it. If you can get me a code number- even tomorrow- I can give you a much more detailed description of the problem.