You have to be careful when measuring votage on an O2 sensor. It is best NOT to disconnect to test it. If you need to see how the sensor reacts in a "live" situation, it is best to TAP the signal wire then induce a simulated lean A/F mixture by loosesning a vacuum hose while watching the voltmeter, the voltage should drop and stay relatively low until you reconnect the hose. You might want to spray a little carb cleaner into the intake and then watch the voltmeter to verify the voltage goes up and stays there until you stop spraying. These simulations should cause a change in front O2 sensor voltage about 1-2 secs. after inducing them.