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peng12 i now have replaced the 2 sensors and a new cat i do not have the p0420 code anymore i think it was down to the second sensor was past its sell by date .actually when i went to put new o2 sensor on i realised one of the wires to the sensor was broken and no matter how many times it was turned off with a code reader it would come back on within minutes of driving .it just took time to realise what it was. I suspected i would have to replace rear sensor because of age anyway .
I should have sorted it sooner as it was dragging my mpg down.Acceleration was a little restricted because of it ,when sensor was replaced you can notice straight away idle was a little jerky before and it accelerates much better, more responsive whereas before it would be a little hesitant
i would say it is one of your sensors causing this as with a new cat it was coming on until the 2nd sensor was changed