Toyota Repair: P0420, vacuum leaks, oxygen sensors

Question
"With the car running between 40-50mph The forward sensor voltage varies between .155 and .855v indicating the rich/lean cycles are in process and the O1 sensor is working normal.  The rear sensor varies between .356 and .755V with an occasional dip to .155v indicating the converter cannot sustain the oxygen level and is probably weak.

My question is: When I let off the accelerator the front sensor reads .000v temporarily then returns to normal readings.  This behavior is mirrored with the rear O2 sensor as it too reads .000v temporarily.  Is this normal operation?  Should the mixture be rich enough to max out the oxygen sensors?  If not, what should I look for outside exhaust and vacuum leaks?
Hope you had a very Happy Thanksgiving - Jack"  

Answer
Those readings are normal when a cat converter is failing, the rear sensor should not vary much at all and should be pretty much a flat line voltage reading, I would recommend replacing the converter.
Please include year/model and what engine in a follow up if necessary.