emmisions workshop + engine management HELP

hi guys ive got a superb 2.0 140 bhp bss engine i had a emmisions workshop light come up and engine management light come on having said that every thing seems to be running ok i got the guy i use to clear the fault and its lasted about a week and its come on again but everything seem to be running ok i did not get to see the fault as he cleared it before i could see it any ideas i willget him to do another scan tomorrow but im at a loose end anything i could check fanks