Is there a CE electronics PI driver board? Unplug it, it fails often. If not, read DC volts from CAN high to ground and CAN low to ground. You should have about 2.5 vdc to high, slightly less to low, & they should both be fluctuating. If you see anything besides that, separate all the CAN nodes on the channel to see where the problem is coming from. The door board and the selector are nodes on the channel too. If it’s a Mod with the HDLM or Smartech II door operator that have the expansion board with UIT plugged in, that’s a CAN node too and I’ve seen it take the CAN channel down. The door will work fine with that disconnected. I’ve seen a power power spike kill every single device on the CAN channel 2.