The ATC light was a steady red on returning to the car. It happend after a 12 hour ferry crossing. The 5 minute reset worked for 45 mins then it came up with error message in the app and the LED returned back to steady red. Once the unit had been plugged in for approx 12 hours towing it cleared itself completely and has never happened again. I have towed 1000,s of miles since and it has never missed a beat.
The point I was making is that the bluetooth connection alerted me to the fault. If it had not been connected I would have been towing thinking I was protected when I wasn't.