i have learnt a lot here
use to have the ffg DTC code P0304, P0300, P0141.
serviced the injectors and it took care of the misfires engine runs smoothly.

Replaced the lower O2 sensor as the former one has been mutilated. so it took care of P0141

After driving this CEL came up and on checking gave P0420 which has never been there.

How do i solve this?

GAZZUZZ:
will be nice to know what vehicle and engine type the catalyst code came from.

Its a toyota camry 2000 model.
its an I4 engine 2.2 litre capacity.
Thanks Gazzuzz

depair:
i have learnt a lot here
use to have the ffg DTC code P0304, P0300, P0141.
serviced the injectors and it took care of the misfires engine runs smoothly.

Replaced the lower O2 sensor as the former one has been mutilated. so it took care of P0141

After driving this CEL came up and on checking gave P0420 which has never been there.

How do i solve this?

It is likely your car catalyst is absent or defective and the reason u didn't get this code all the while is that you now have a working catalyst monitor sensor (o2 sensor 2) which is working now and can detect faulty catalyst.
But I wont advice you to just go ahead to fit a catalyst until it is 100% confirm the catalyst is not available cos of the cost and beside not all p0420 is actually due to a faulty cat.
Go for further diagnosis to confirm the catalyst .
If you come to my workshop I can help you confirm the condition of the catalyst without bringing down the exhaust within a few minute.