For the record on this problem. The engine light came on and we 'home checked' with our OBDII Diagnostic Tester and it showed 0420. Our first visit to a Jaguar expert resulted in a replacement of one of the oxygen sensors in the Catalytic converter. That fixed the problem for about a month then it came back. That cost $250 dollars!We learnt from noting the characteristics of the vehicle that the Catalytic converter was not blocked and one of the tests is checking for smooth acceleration from 0 to pedal to the metal. However that is rather difficult on regular roads so we had to go into 'assumption mode' after realising the car,of which we call 'Ruby', was responding like a young jaguar anyway.We have also learnt that not all premium fuel is equal and we found a listing of Gas Stations in our area for the most reputable and therefore best fuel.Also we were introduced to a product called 'Seafoam' which is meant to be and upper cylinder cleaner and does not harm the catalytic converter. At least, that is what they, the makers of the product say, so:1) We have ensured the best quality fuel to fill Ruby.2) We are experimenting with 1 pint of Seafoam in a minimum of 8 gallons until we fuel up soon.3) After resetting the code and putting on approx 100 miles we have been encouraged by no engine light. A light mileage as yet but it is nice not to see that engine light.

 

There are several threads on this forum regarding OBD code P0420 and the various remedies that have been tried.

From my personal experience, the least costly method was to replace the RH (Bank1) catalytic converter.

If the downstream oxygen sensor is replaced and the code reappears shortly along with the MIL coming on, the catalytic converter is likely no longer able to function correctly.

 

You can replace every sensor on the car but basically, your catalytic converter is no longer functioning as it should and the O2 sensor is picking up the data.


Replace both front cats. They're not that expensive. Check www.rockauto.com.