My engine warning light came on yesterday but it went off after a bit but its done the same today so i plugged a computer to my ford and it said the o2 sensor has a problem. Wat usual causers theirs to break and are they expensive to replace?? Thanks

At a guess, i'd say that you have a Catalyst problem unfortunately.

Maybe wrong though, needs more investigation.

The sensors can just fail, you have two on the st one before the cat and one after the cat so find out which one is causing the problem, you can buy universal sensors and i have used them before, or buy an original ford sensor, I have just ordered one for an audi a4 universal for £18, an original will be a lot more though.
http://cgi.ebay.co.uk/LAMBDA-SENSOR-FORD...=item4150a970f6
this one is for the st.

Cheers guys ive rang up the garage i got it from cos i aint even had it a week yet and iam taking it back to him next week and he is fixing it for mme