All of these tests are much less stringent than the original standard the car is built to. When I am testing these cars with an exhaust gas analyser I only believe my readings for about a minute at the most. A car with a good cat will read 0.0 CO as soon as the cat is hot BTW it some how doesn't seem strange to me that a 15 year old cat wouldn't work as good as new.
We brought a few cars in from Germany (grey market stuff) in the early 80's. We had to bring the car up to the standards of the federal test sequence; much harder than ASM or IM240. We had to haul the cars to a test lab in Ft lauderdale. The test lab was in an industrial jone and the tests were so tight that back ground readings were taken to justify the actual readings. We had one 500SEL so clean that on the warm bag we cleaned up the hydrocarbons from the background. In other words the air coming out the exhaust was cleaner (for unburned hydrocarbons) than the air we were breathing.
Bosch Master, ASE Master, L1
33 years MB technician