Ok,you probably won't like my answer very much but here goes,the very best diagnostic equipment in land is let's say misleading.All engine sensors are in what is called a "closed loop" system and are all feeding information back to the E.C.U electronic control unit to regulate the emissions.Closed loop system means all the sensors are linked to each other, they all compensate to a certain degree if another sensor becomes faulty and there lies your problem is it just a faulty sensor?or a faulty cat?or has another sensor failed causing the emissions to be out and this is what is putting the light on.The garage i feel have advised you as best as I feel they can because nobody can say 100% it's this or that.Personally if the engine is running ok,no misfire/lack of power etc I would take an educated guess at it being a faulty sensor.Sorry if that doesn't help much but there is no diagnostic system that will tell you it definitely one way or the other.