Hi Adrian,

Your pre-cat Lambda reading is too high and not switching between 200 - 800mv every second (1htz). This needs to be dealt with before the cat is changed. The cause could be faulty Lambda sensor, or poor Lambda earth. I am assuming the rest of the engine management system is ok. Sometimes taking the sensor out and giving it a good clean might do it. If you have a poor earth (voltage above 0.25v) then tapping in a direct earth will do it.

Regards

Pete M