Just checked back into my ALLDATA CDRom on my '92 to verify just what the procedure really is to check the O2 sensor. It DOES say to use the DC scale, NOT the AC scale. Ground one v-ohm meter terminal, attach + DC side of meter to o2 termineal, with wire disconnected. Manually create vacuum leak and/or introduce propane into intake and check your DC readings. Will fluctuate from about .1vdc to .9vdc.
When I used AC scale I can not get a stable reading. When I use my ocilloscope on AC scale I get too much interference; again the DC scale on the ocillo works.
The fact that YOU are taking a reading downstream with the sending wire connected may just call for that AC reading, but my manual does not cover that procedure.
2001 SSEi- black; stock 92K
1995 Vette; Competition Yellow; stock; 38K