just had a very similar problem with a 52 plate focus recently, fitted new cat. fuelparts.(old one collaped-no argument there) cat efficiency code re-occurred intermittently, checked everything i could think of, turned out after speaking to their tech. department that o2 onwards the programming for the warm up cycle on the cats are shorter & sometimes the cat. can't complete the cycle in time. they are "recently aware" of this problem & have a new part no. that will comply, more money of course! job done tho. lost hours & hair.