Usually a vacuum leak.

PCV hoses and intake manifold gaskets are really common sources. Could be a intake hose leak or a MAF sensor issue.

Click on SEARCH, select Troubleshooting forum, keywords "P0171 P0174", select AND, select ALL POSTS, and see the results of numerous previous discussions.

Steve

98 Contour SE Sport 2.5 Duratec ATX The wifey's car 89 Taurus SHO - 246K miles 94 SHO ATX - 190K 1997 F-150 5.4L ATX - The Workhorse 150K. ANY THREAD WITH "OMG" or "WTF" ETC IN THE TITLE WILL BE IGNORED!