a1115178_tar_Data_API_Developer_s_Guide__Explained
[Error] 503 [Explained] 
Service Unavailable, returned when the API is unavailable to handle the request due to a temporary overloading or maintenance of the server. The existence of the 503 status code does not imply that we will be using it on a regular basis. It is here to cover rare cases only so that we can message and inform consumers if needed.
GuideStar Data API Developer's Guide
Explained