I'd start with changing the fuel filter. Attached page out of 2007 Shop Manual lists possible causes for P0171. I've never had this fault, so others hopefully will chime in that have 1st hand experience.

If you haven't downloaded one of the Service (Shop) Manuals from the website, you really ought to do this so you have one available to review whenever something like this fault pops up. Any of the manuals will do that are listed on the Forum. (Note: If you have a Roadster, you'll need the 2005 or later version. There were no Roadsters in 2004, so it's not covered in that manual.)