I was just looking through an old manual I have for System 3. The CP-30 will light the trouble lamp if there is a ground fault anywhere on the system. There’s a ground fault jumper on the CP-30 to the right of P1 (a two pin connector). Remove that temporarily & see of the panel clears. If it does, there’s a ground fault. If no grounds, hold down the reset & silence buttons, remove the system buss connector from P1 to disconnect the modules and move the connector from the top left module (the one with the single wire) back to the CP-30. This will isolate the modules from the CP-30 to see if it’s a bad module or CP.
Also, the ZN-30 is NOT compatible with the CP-35, so you will need a ZU-35. Is that a ZN-31 in there too? If so, not compatible. It looks like only 4 zones, so you would just need one ZU-35.
Is there a way to upload or attach PDF’s? I can scan the cut sheet & post it (I have all the cut sheets for that panel).