The vast majority of the problems were a system that missed the time window to report it's self test. A reboot got things in order.

It was a small fleet, so we had a pretty good idea if the same problem kept popping up so we could look more into that system.

A few fault codes were well known enough that if you called with that one the answer was to shut down and find a hotel, you aren't going anywhere and no reboot will take care of it. (bad detectors in the engines for about 10 years until a redesign)