How to replace p0420 and p0430 sensors - 2002 Jaguar S-Type

P0420 and P0430

P0420 means Catalyst System Efficiency Below Threshold, Bank 1

P0430 means Catalyst System Efficiency Below Threshold, Bank 2

It means that the O2 sensors are good, and the O2 sensors are telling you that your catalytic converter, in the exhaust system is not working fine, as its emissions are polluting the air too much.

The catalytic converter is part of the exhaust system. The way to test this part is by doing an emission test. The two OBD codes that you are getting are already a test telling you that the converter must be replaced, unless an emission test done at a garage says the opposite.

This kind of problem will trigger the check engine light, and your car will also waste too much fuel.
You also risk to get in trouble if the car goes through an Air Pollution control, as the car won't pass an emission test in this condition.

To conclude, the Catalyst System must be replaced, the O2 Sensors are fine.

This is a repair that should be done at a garage, however you can find DIY instructions here:How to Replace a Catalytic Converter or you can get more detailed instructions, and step by step illustrated procedure purchasing a repair manual.