petrol 2.2; code P0420 recurring

[terry paget]

petrol 2.2 manual saloon
Bought this car in Neath e-bay £190 2 weeks ago. EML was on when purchased, read code, P0420 catalyst on bank 1 not working well, cancelled code, all was well for 600 miles, Yesterday EML came on again, same code. What might this be, how can I prevent recurrence?
I imagine the ECU info came from the second lambda sensor in the cat section, used for German MOTs. The mass air low sensor on car was non-Vx with poor socket which would not latch onto cable plug, so I swopped it with a spare, but this too was a £30  aftermarket item.

[biggriffin]

Fill with super unleaded, or shell nitro, and thrash the living daylights out of it.
Normally works.