Got a friend that took his truck to Autozone for a code check and they came up with P0406. Any ideas on what could be the problem? I don't know anything about the 4.2 engine, but have recieved great info here on my older Ford trucks. Hope you all can help out. Thanks in advance..

 

P0406 Exhaust Gas Recirculation Sensor A Circuit High

If you do a google search on "Ford code P0406" or similar, you can find more info.
I'd suggest buying a Haynes manual. They are pretty good and I seem to remember them give info on testing suspect components for various OBD II codes.

 

It's probably the DPFE sensor, this tells the ecm how much gas is flowing. Mine went bad, it cost about $35,and took about 5 minutes to replace.