It has been observed when:
APAR: RO71806 Title: MARK VT UNAVAILABLE WHEN VTAM ISSUES RC=14-13 SENSE CODE is applied, this can be a possible cause of more virtual terminals (VTs) marked unavailable in TPX 5.4.
With this fix is applied, TPX marks a VT unavailable for this condition (RTNCD=14 FDBK2=13 MEANING=CID INVALID). A flag is set that will cause the VTs to be checked at the INQUIRE CHECKING INTERVAL (SMRT) to see if they can be made available again since they are not permanently marked unavailable.
If INQUIRE CHECKING INTERVAL = 0, no checking is performed and the VTs will remain UNAVAILABLE until TPX is cycled or individually marked available with TPXOPER (MOD VT=vtermid,AVAIL).