Originally Posted by 97 SL320

2,000 MV = 2 V

O2 sensors make 0.500 to 1.000 V when the exhaust is rich ( low O2 content ) , 0 to 0.500 V when the exhaust is lean ( high O2 content )

This type of O2 sensor does not have a linear output. The cross over point between rich / lean is very narrow. There is no way to tell if you are slightly rich or fully rich.

When the sensor fails, voltage drops , the computer thinks the motor is running lean then starts to add more fuel. Do you ever see more than 300 - 400 MV ?