When cayenne can't call the device and talk with it. It will say the command cannot be sent to let you know from Cayenne's side there is an issue. However, lets say cayenne talks successfully to your arduino, but hte arduino had an issue actuating a relay. Cayenne will not know that, I don't think. That's something I'm working through in testing now. I'm powering the 5v relays from the arduino board itself. I know that in the long run i'll need to provide a dedicated power source for the relays, but this is testing/discovery. When i stress the relays through many actuations of the relays together, sometims they will stop responding and 'drop out'. The arduino will still report all other sensor data, and i can click a button on the dashboard to actuate a relay(and the dashboard will show a successful command sent), however in real life, the relay is not kicking on. I think that's where you would need to devise a type of watchdog program that would reset the arduino. Though i'm not an expert with experience in that area. (i'm interested in this too as I'm thinking through that process.