Fault Code P0420 Fiesta MK6 1.25

[Augite]

Hi,

My 2003 Fiesta was displaying a yellow engine management light, the person who sold it to me told me it was a faulty lambda sensor. Shortly after, i was experiencing problems such as rough idle, loss of power and sluggish acceleration. I got the car to a garage who fitted a new lambda sensor after they diagnosed it.

For a few weeks the car has been running sweet as a nut, but recently the engine management light has come back on while driving on the motorway and using a OBD2 sensor I received a P0420 Catalyst system efficiency below threshold (bank 1). The car idles roughly sometimes slightly.

What would the problem be for my Fiesta, and how and where can I fix it?