had the same problem with my 90 300SE a few years back.
It had high miles on it and the MB replacment Cat had about 80K on it.
First test was:

Grams Per Mile
Reading Limit
HC 1.06 2.00
CO 5.86 30.00
NOx 4.17 3.00
CO2 359.26 N/A


It was in Aug and was running hot, found that the aux fans were not on and the water temp was about 100C
Got the fans working again.
Found a few rubber elbows for vac lines that looked cracked so I replaced them. Also changed the oil and plugs. The plugs only had 10K on them but they looked tired. Also added a gas additive called Guarantied to Pass I got at napa.

Took it in for the 2nd test and the guy there said no way it will pass, it passed and NOx went down to 1.2

__________________
1969 RS/SS Camaro Conv. L-89 Alum. Head 396 375HP 4-spd

1987 GNX #419, 2002 RX300

FS: 1934 Ford Street Rod, Blown 383 $48,500 Firm

http://banners.wunderground.com/bann.../worcester.gif