I read your first post that the issue you were having was with the Filter pump, and that the water feature pump was OK>
Jim has you on the right track.
In theory, as long at the Pump #2's display says "Display Not Active", and the "Water Feature" circuit is on, and the System is in "Auto" then the pump should run.
If it does not run, then an RS-485 communications issue is most likely the problem..
It could be the cable
It could be the RS-485 chips in the Pump
It could be the RS-485 connection between the cable and the pump.
What I suggest is that you ensure the connections between the RS-485 cable from pump 2 is making good contact. RS-485 has different send and receive chips.. It is possible that only one side has gone bad, (or come loose) and that is why the pump says "Display not active"..
Since pump number 1 is working fine, as a test, I could disconnect the RS-485 cable from pump 1 and connect it to pump 2.. (Just right at the pump, not inside the ET..) If pump 2 then runs you would know it is the cable or some connection issue. If pump 2 still does not run, you will know the pump is the problem..
Note that you should never connect or disconnect any RS-485 connection with power on as it can damage the chips.
Based on what I see, you are right. It's something in that cable or the chips. What is odd is that the pump came on randomly when I was not attempting to work on it. Given the service mode works, it has to the cable or the chip.
I will try the option to swap them out and see what happens.