It means that the cat is not functioning sufficiently to reduce the emissions to the legal level. The elements in the catalyser need to be extreemly hot to function well and obviously there is a limit to the quantity of CO etc that it can cope with. One thing to try is exhaust wrap to make sure the cat receives gas as hot as is possible but there may be other problems with the state of tune of the engine. This is a very common issue with sports exhaust systems.

Peter

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