[s-cars] New acquisition - basic questions

[Radek]

Hello everyone;

Through my son's recent purchase of a 1996 UrS6, I'm becoming involved in a car that I'm not familiar with.  Nice machine, but seems to be running rich and not idling very well.  Sometimes drops to 800 rpm, sometimes stays above 1000 rpm.  Also, persistently throws Engine Speed Sensor G28 code:

00513 - Engine Speed Sensor (g28)
            03-00 - No Signal

I checked the sensor for resistance and it's OK - about1000 ohm.
Any suggestions where to look, apart from cleaning the ISV and changing the oxygen sensor, which we are planning to do next?
Thanks.

Radek
91 V8Q 5-sp
88 90Q