buy a code reader off ebay. my st170 did the same thing so i bought a code reader off ebay 22 quid. found the problem to be rear lambda so then got that off ebay 21 quid. problem sorted and now have a code reader for future problems on any car. 43 quid in total to sort my focus out and been ok since, its your best option and to do the same rather than pay a garage say 30 quid to tell you the lambda is fooked