Sounds like normal operation. In order for the catalytic converter to function correctly it needs a constant high and low of CO in the exhaust stream. That's what you are reading on the front oxygen sensor. The exhaust at that point should be switching from lean to rich and back (0.1v to 0.9v) around 60 times a minute. Now after the catalytic converter, the exhaust should be cleaned up of carbon monoxide (CO) and voltage should stay close to a steady 0.4v.