Found the problem - the EGR sensor I used had failed in the 100 miles following the repair. I bought another sensor from NAPA - they were good enough to allow me an exchange on the 2nd sensor if the 3rd solved the problem. The key is to measure the voltage across the sensor with the ignition on, engine off. I used stick pins to make contact with the connections. The good sensor read 1.2V, the bad one read .16V. Everything below .2V is allegedly bad. This one threw me because I had not experienced a bad replacment part before. The truck has now gone 2 weeks with no new check engine light

 

the differential pressure feedback egr sensor or the "DPFE" sensor fails on rangers all the time causing the check engine light to come on. so that sounds like what you got going on. on f150's it's probably the same set up. it just senses the pressure moving through the egr. one for the ranger costs around 80-100 dollars depending who makes it, and you can install it in like 5 minutes with simple hand tools, at least on the rangers you can. a 1/4 in hex socket, and a small slot head screwdriver should suffice. disconnect the positive terminal on battery, unplug sensor and then unscrew it from mating surface, install new one, plug in. hook up terminals, and wait for engine light to come on. most say it stays off. i'm trying this before i go to a smog test.

 

2002 mercury mountaineer service engine soon light is on ,what should i do?