Hi, I just bought a Ford Focus 2013 1.0 ecoboost last week from London and I was on my way to Newcastle, just 40 miles away the check engine light came on. Today I had it checked with Halfords here in Newcastle and one of their guys said that the catalytic converter is faulty and need to be replaced which would cost something around £600. The car's MOT has just been done this month and I am confused as what could have gone wrong. I have got a warranty first cover as well but I am not sure if this will be covered.

The engine ECU checks cat efficiency using the lambda sensors.  The MOT test allows a wider acceptable range than the cars sensors do.  So it's perfectly possible for the cat to be out of range for the ECU, but still pass MOT.  However, it is also possible that the lambda sensor is faulty.

Generally, this sort of thing isn't covered under warranty.  But as you've literally just bought it (presumably from a trader?) then hopefully it will be covered one way or another.

Quite typical problem for 1,0 Ecoboost. Mine cat (2016) died at around 55 K miles but I have the light on only at high speed on the motorway. Within the city I may drive miles without any warning light. For now I just delete the faulty code. Unfortunately these cats are quite expensive! 

Did Halfords remove the fault code for you? Might be worth giving it a few days to see if it comes back on. That's a long return trip back to London if has to be done there under warranty.

I note your car is over ten years old, has the cambelt been previously replaced?