I ask the same thing everytime is smogged and resmogged. The tech guys basically don't know the reason, but say that it could be the radio. They say the radio on some cars is integrated into the OBDII or computer or who knows so that when it is removed, the computer for some reason or another will not communicate with the smog machine.

The smog test results
pcv pass
cat conv pass
visual egr n/a
fuel inj pass
vacuum line connection pass

MIL - FAIL

fuel evap test n/a
thermostatic air n/a
pump air inj pass
ignition spark controls pass
o2 sensors and connectors pass
other emission related pass
functional EGR n/a
Fill ppipe restrictor n/a

fuel evap controls pass
fuel tank cap visual pass
carb n/a
wiring of other sensors or swithces pass
ignition timing TDC n/a ?
liquid fuel leaks pass

The idle emission test results all passed with a wide margin.

The only fail was something called MIL

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1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.