Late eigthies/early nineties Mercs appear to be very sensitive to cat temperature. You MUST make sure that the car is thoroughly warmed up - say at least ten miles or 20 minutes of urban driving. Do not shut the car off prior to the test. In fact, run the engine at 1500-2000 revs while you wait.
The converters slowly degrade with age/miles, and loose margin. The hotter the converter, the more effective it is, and this becomes critical to passing field emission tests on older cars.