Awhile back my car engine light come on and I took your advice had it checked and reset the code (po420 catalyst system efficiency below threshold) now it came back on but just so happened it was having a service the next day, so had the sensor changed on the cat now 2weeks later the engine light is back on ? Any ideas ? Now cars going back in tomorrow for them to check over, but I'm getting fed up now of the same problem occurring

Any ideas ?

Sam