This seems to be problem on some Futura controllers. I have the same issue on one job minus the Fire service condition. Many times you will walk into the MR and find a car shut down on a 21 error which usually indicates just as sbrmilitia stated. If you type in a CMC command it will show everything communicating. If you type in a GET command it shows up as OK. We have changed out the SPU power supplies which seemed to be CEC's fix for the problem. In a few cases I have replaced the SPU board itself due to faulty eeproms causing the problem. Of course now with CEC being out of business this is even tougher. This job is about 10 years old and it did seem like CEC had some software problems about the time the code was undergoing pretty severe changes early in 2000.
