Those CAT motors will always be trouble, just the environment they live in and not a great system design. There also using poppet valves to control the heat in the exhaust and they have had many problems

You need a scanner or Diacom software to find out anything that is happening. Or need a certified tech, because if the CAT gets the wrong mix it will be damaged and then its $1500 a side.

Ouch! Thanks for the information. If the pre cat O2 is reading fine, but the post cat O2 randomly goes to zero is that a true bad mix? Just trying to learn as I go....

I'm no expert, but it reading zero would have me looking at the wiring, maybe loose connection. The first thing I would do is get the motor scanned