Its proberbly a lamber sensor mate, st170 eat them. If the car is running fine and if you want to clear it disconnect the battery for 5 mins and it will reset but i would get it checked 1st.
If it is lamber sensor dont buy from ford go on ebay.com in the USA and you can get both sensor for a fraction of the price.
Hope this helps let me know how you get on or if you need any more help pm me.