PO401 error code question

Hello aaf group,

         I have a 2002 Explorer with check engine light coming on.  My
local 
mechanic tells me that the Code is PO401.   The nearest Ford dealer****p is

about 50 miles away.  My mechanic says that there is a list of stuff it 
could be but he dosen't know which item is the problem.  Can anyone tell
me 
what the problem may be or the most likely it could be?

                                                                          
  
                                                                          
  
                 Thanks group , Ed