Catalytic converter (CAT), Catalysator, Katalysator

This is the original ceramic based CAT used in the 928 staring MY 1986/1987.

One of the problems is that it may get clogged (breathing problems in the engine) or broken (ceramic breaks).

In Germany it the Liphardt metallic CAT is often indicated as a better CAT, also to get better inspection grades (EUR2) and thus lower taxes. You can see the tubes that enter fresh air from the air pump. A system that is used in Porsche 928 with CAT blows secondary air directly into the CAT. This measure is required to keep the CAT cool, and burn remaining HC particles inside the CAT and not in the beginning of the exhaust (some brands inject air into the exhaust before the CAT).
Without the secondary air the CAT could not perform the HC conversion at part or full load.

There is a check protocol that says that at TüV (technical/environmental inspection) the diverter valve vacuum line must be taken off to prevent air entering the exhaust and CAT, and show a lambda value around 0. If the extra air is injected the lambda value will be off due to excess of O2 in the exhaust. In the Netherlands the inspection for Lambda value is not even mandatory for a 1992 928GTS (not sure at what MY it starts).

 

This is a proper x-pipe with metal CAT's: