Diagnose the cause of the problem that led to the catalyst being changed and make repairs before fitting the new catalyst, otherwise it will not work properly. Possible causes are: excess unburned fuel and overheating in the catalyst, misfiring, ignition system failure, towing, malfunctioning spark plugs or spark plug wire (petrol models). Other causes may be oil or antifreeze entering the exhaust or sealing compounds that may have contaminated the catalyst core, lambda sensor failure (gasoline models) or broken exhaust system hangers.