Ingo,
I looked at the schematic very carefully this time and it shows a voltage divider network of 4 resistors (R802,803,804,805) that setup 3 reference voltages as follows:
Rich - 0.63v
Target - 0.6v
lean - 0.57v
The rich ref voltage is wired to one comparator
The lean to the other
And the target is feed back to the O2 sensor input, doing this insures that if the O2 sensor is disconnected then the output from the 2 comparators is guaranteed to indicate target lambda. From what I see it's perfectly OK to disconnect the O2 sensor without any fear of the lambda control getting in the way. Simply if the O2 sensor is not connected the comparators will always indicate they are at lambda.
Hope this makes sense.
Also, this is from the 944 but the basics are the same in the 911 just the voltage divider resistors are slightly different values in the 911
http://www.the944.com/lambda.htm
Great write-up by frwilk
Also he took liberty to re-draw the schematic in a much more readable fashion than the 911 dme schematics.