You have a very marginal failure. There may be nothing wrong with the emission control system, and it may be just a case of failing the somewhat arbitrary NOx cut point.

There are a couple of things you can do, and both involve retarding the spark advance map, which will reduce peak combustion temperature, which can dramatically lower NOx.

First, temporarily plug the vacuum advance line to the EZL.

Second, replace the R16/1 resistor with a shorting plug that you can make from short piece of wire. This will minimize the rate of advance with engine revs.

Search the archives if you don't understand what I mean by R16/1 resistor or where to find it on your model.

Retarding the spark advance map will reduce "engine out" NOx due to the cooler peak combustion temperature. It will also increase EGT, which will keep the catalyst bed hotter and more efficient.

Be prepared for a loss of performance, particularly below 2000 RPM. The above two temporary changes will reduce power so much below 2000 that it will be a real dog, but it's only temporary until you pass emissions.

BTW, if you do not replace the OE R16/1 resistor, which is probably 750 ohms - just leave the socket open, the rate of advance with engine revs will be more aggressive than OE, and the engine will have noticeably more low end torque, and probably reduced fuel consumption in around town driving, but be sure that it's not accompanied by detonation.

Duke

P.S. I have a background in automotive emission research. If you search the archives under my handle, you will find a number of threads where I explain how emissions are created and controlled, how to MANAGE your emission test, and low cost, easy ways to improve the emission performance of M103 engines based on experiments and tests I conducted on my '88 190E 2.6 five-speed that I bought new and still own.

My car (as is the case with most M103s) was always close to the HC limit in the California ASM 15 MPH test, which precedes the 25 MPH test. The primary issue is probably normal catalyst degradation due to aging. The above two changes dropped HC by about 40 percent due to the hotter catalyst, which gained me a comfortable margin. NOx was never a problem, but the lower peak combustion temperatures reduced engine out NOx so much that the tailpipe reduction was about 90 percent!!!