I happen to have done chemistry, so don't need to. The only way overall car emissions could have reduced 96% net is if fuel consumption reduced 96%, which it hasn't, otherwise average mpg would be 500mpg.
Catalysts actually change C & CO into CO2 as it's slightly less harmful. It also converts NOx to N2 and O2 and unburnt hydrocarbons to CO2 and H20. The CO2 emissions are still there. Maybe CO emissions are reduced 96%, but it is quite wrong to say that is the net result.
There is an exhibition at the national car musium comparing a Ford Cortina with a Mondeo. It states that the emissions of the mondeo are 96% less then the Cortina. Try writing to them if you disagree