Are you saying you disconnected the O2 sensor signal line to measure the voltage? That's a no-no.

The voltmeter must be installed parallel to the circuit without altering the circuit! Pull the O2 sensor signal lead out of the socket enough to connect the voltmeter probe without interrupting the circuit. Since it's a rather long pin, I've found this easy to do with some extentions that have very small clips at the ends, so I only have to pull the pin out about 1/8" inch. If you get a check engine light with an O2 sensor code, you've interrupted the circuit.

Use any good ground and the battery negative terminal is fine.

A digital voltmeter should jump around a bit near 450 mV.

If you have a scope the voltage should oscillate between about 0.2 and 0.8 at a frequency of about 0.5 to 1.5 Hertz.

Duke