LISTENER
Check for non-successful connection attempts in listener.log
How to Purge Listener Log Data
Listener Error:
ORA-12162: TNS:net service name is incorrectly specified
ORA-12505: TNS:listener does not currently known of SID given in connect descriptor
ORA-12514: TNS:listener does not currently known of service requested in connect descriptor
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor (DGMGRL)
TNS-12516 TNS:listener could not find instance with matching protocol stack
ORA-12516:TNS:listener could not find available handler with matching protocol stack
TNS-12519:TNS: no appropriate service handler found
ORA-12519:TNS: no appropriate service handler found
ORA-12520:TNS:listener could not find available handler for requested type of server
ORA-12528: TNS:listener: all appropriate instances are blocking new connections
ORA-12154: TNS:could not resolve the connect identifier specified
TNS-12547:TNS:lost contact, Linux Error:104: Connection reset by peer
TNS-12537:TNS:connection closed, Linux Error: 29: Illegal seek