Oxygen Sensor Voltage

[96C280]

Using a Snapon Mt2500 Scanner, I activate the secondary air pump in closed loop. Is the voltage supposed to drop to 40mV and stay there? My results are not consistent. Voltage before the activation can be anywhere from 150 to 4-500 mv. The first activation seems to drop the voltage the most but may hit the 40 mV only momentarily before bouncing back up to 150 -200 and stays there. Is the ecu compensating for the lean condition. When attempted again (maybe a minute later) the drop is never that much and does not approach the 40mv. Any ideas what is going on? The pre-cat O2 sensor is OEM and was replaced not long ago.