From the manual:

"@1988 California and
@1991 with diagnostic trouble code
storage:
The CFI control module must be switched over
to output of on/off ratio because the signal for
the diagnostic trouble code (DTC) display is
output as priority.
The output of the on/off ratio is performed once
all the DTCs have been read out and the start
button of the impulse counter scan tool or the
non-locking switch on the diagnostic connector
(California version only) is then once again
pressed (see section m).
The % readout now fluctuates when the engine
is running at normal operating temperature and
there are no DTCs."

I built a homemade LED Tester to put my '93 190E into diagnostic mode to read duty cycle. It also reads DTC's.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible