P0420 relates to catalyst efficiency - the rear oxygen sensor is reading too closely to the front - which in effect means that the catalyst isn't doing its job. Sometimes poor fuel quality I've known bring on this fault, but ultimately it'll mean a replacement catalyst will be required.

 

Read the link below first chap before you go down the new cat route, generally a cat will last as long as the vehicle providing your engine is functioning correctly, quality of fuel has little difference to be honest, its how its delivered and how much is being delivered and ignited. Little things like worn sparks plugs, air filters, mafs sensors, temp sensors even front o2 sensor or vac leaks etc can cause all sorts of rich conditions, these need to be addressed before a new cat is fitted otherwise it will just fail again.

http://www.easterncatalytic.com/education/tech-tips/the-misleading-nature-of-the-po42o-code/