P0420 Code is the rear lambda thinking that the cat is not working............It's unusual for this sensor to break as it's pretty far downwind from the engine.
Hopefully the problem was the broken wire and it should go out after 5-6 starts when the new sensor is fitted. Unfortunately the Bugeye ECU is a bit flakey on the P0420 CEL and Alyn may have to re-set the ECU for you
Cheers
Shaun