When you see a task waiting EXT ECB or a task abends with a message
"DC001003 Vnnn Tnnn TASK:taskcode PROG:pgmname STALLED WAITING FOR EXT ECB AT <addr> "
it means that IDMS could not identify it as one of the External ECBs we know about.
This typically means the User Application has established its own ECB and has code that #WAITs and #POSTs that ECB.
It is possible that a Task Snap from the DC001003 might show the ECB the task was waiting for but only if that ECB was in storage owned by the task.
The address of the ECB is in the DC001003 message so if it is an ECB in some kind of control block that is permanent or still allocated at the time you look, you May be able to tell what it is.
IDMS deals with two types of ECBs. Our IDMS internal ECBs and the standard single-word External ECBs, and we have a list of "types" for all of our internal ECBs and all of the External ECBs that IDMS system software deals with.
. You probably need to capture a SNAP to determine the offset in the application where the #WAIT is being issued. Once you have the offset you could then go into the application to determine what the ECB represents.
Additional Information
The #ELEDS dsect contains the list of external format ECBs IDMS recognizes.