if you wanted to disable it and not take the dash apart you call randy over at Focus Sport and order an sct xcal. their website is
FocusSport - Performance parts manufacturer for Ford Focus they are over in california and do alot with the focus. the best thing about that is if ever decide to mod your focus power wise you can use the sct to better help the power gain. also if you decide to do exhaust and header on the car it will eliminate the check engine light you get from it. but if your not wanting to mod your focus the $385 it costs might seem a little steep for you to just get rid of a light on the dash. if you have any questions about it just ask im pretty good with modding a focus.