We just took over a new account. System is a Trane Voyager 30 ton with inlet vanes. There is 17 boxes, some parallel fan boxes, some series, all with heat. When we arrived the CCP was in priority shutdown with no communication to any device. we discovered that the bus polarity was reversed at a junction box above the ceiling?? Once corrected we were able to communicate with most of the boxes (all but three). We fixed one box that we could not get to communicate by replacing the UCM. Polarity was reversed at this box also. With a section of the box bus disconnected (about 5 boxes) the CCP will intermittently communicate with the unit. The CCP will display fan status and supply air temperature, compressor status, etc. for 10-15 seconds then the info changes to question marks. After about a minute the information is diplayed again and drops off again. The unit stays onlne. The first tech replaced the TCI board in the unit under warranty. Trane had replaced it a short time ago. This made no difference.
When we found the polarity issues we went to every box to check bus polarity and addresses. We found no other issues. We did notice that the communication bus was wired to the sensors. This wire is not shielded.