Hi all new to forum, can anyone advise of diagnosis please. Bought 2.0 se passat 2 months ago from non vw dealer. After 2 days engine management light came on code P0420 recorded. Returned to dealer, lambda sensor replaced, light out. A week later, difficulty starting, engine flooded, engine misfire, fault code P0301 recorded. Fault erased, 2 days later, same problem P0301. Retrned to dealer, pistons re-rung. 2 days later original code of P0420 returned. New cat fitted. One week later P0420 code occured again. No loss of power but just this annoying code. Car now back at dealer, any ideas please.