engine management light?

The engine management on my 2001 XTYPE 2.5 petrol manual reg Y208XJU comes on and off intermitantly.I have had this checked by 2 garages on diagnostic equipment and this shows a fault with the catalyst,however both garages do nt think that this is the fault as there is no fault with the emmisions and the individual catalysts show no loss of oporation, the car runs extremly well it has done 95K