You have good information on the service light. There is also a "Check Engine" light that will stay on if there is a fault detected by the ECU. That is usually emissions related if you don't detect any drivability problems.

The sensor is a NTC device of Negative Temperature Coefficient that reduces resistance as it gets hotter. A reading of -188 indicates there is a break in the circuit somewhere. It could be the wiring as well as the sensor.
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'96 965, 16' wheels, Michelin Pilot Sports, rear 18mm bar + Koni, 204HP cams, 126K. Put 200K on '85 745 TD.

Dear Minneapolis960,

Good p.m. Sorry to hear that it's -188 degrees in Minnesota. I'd recommend you install a block heater. I'd guess that the temperature sensor - located below the grill - has failed.

As to the service light, it is supposed to come on, and then go off - to remind the driver, that it is time to get the car serviced, or to do the work. The interval is every 5,000 or 10,000K miles. On the face of the instrument cluster, you should see a small (1/4" diameter) black rubber plug. Remove it, using your fingernails, or the edge of a plastic (not metal) putty knife. Using the tip of a ball point pen, press gently on the button, behind the plug. That will turn off the service light. Then, replace the black rubber plug gently. If you press too hard, it will pop inside the cluster, which will have to be removed, to get at the plug.

Hope this helps.

Keep warm!!!

Yours faithfully,

spook