Thanks for the suggestion. I pulled the OVP relay and found the fuse was good. I also located my long-lost MB Electrical Troubleshooting Manual and now understand the significance of the OVP relay in all this.

Prior to pulling the relay, the car was almost impossible to start. After replacing it, the engine started immediately but the O2 light remains on. The voltage at the battery while idling is 13.84 V.

The sensor has 3 wires. Two go to a dual connector and I presume they are the heater supply lines. At idle, they have 13.5 V across them, which seems reasonable. I couldn't measure the third wire which I assume is the sensor output today but it's the next step.

I understand these sensor cells should be putting out roughly 0.40 V with a stoichiometric fuel / air mixture. Anyone know if this is about right?

Thanks much.