The oxygen sensors should have a range of 0 to .9 volts. 0 is lean, and .0 is rich. The downstream oxygen sensors should switch much slower than the upstream sensors unless the cats atr dead. The downstream oxygen sensors job is to monitor the cats. A cat that works well will have more even readings or you might say that the cats dampen the readings. When the rear sensors switch at about the same rate as the front sensors, the cat monitor in the PCM will set a code and turn on the check engine light. The cat monitor only runs ocassionally after several conditions have been met.
Fix the EGR problem then retest to see what you have.
