A loose temp sensor will give a different error.
The processors get removed when the power supply is replaced. The error you are getting probably means that the service tech reversed the processors (which is OK to do) then neglected to calibrate the processors for their new position. That would cause that POST error. One (or both) processors may need to be reseated, but take your G5 back to the service shop, so they can run the Apple Service Diagnostic (different from your Apple Hardware Test, and the shop is the only place that will have it)
The little right-angle plug is probably the connector for an Airport card (which could be hanging loose if you have not installed an Airport Extreme card), and nothing to be concerned about. (could be tucked back out of the way, if it is just hanging loose.)