Last_IO_Error: error connecting to master 'repl@12.18.46.78:3666' - retry-time: 10 retries: 86400

This error you get when you check " show slave status\G;"

Solution as follows:
Do Telnet:

[root@sj ~]# telnet 12.18.46.78 3666
Trying 12.18.46.78...
Connected to 12.18.46.78.
Escape character is '^]'.
uHost '12.18.46.78' is blocked because of many connection errors; unblock with 'mysqladmin flush-hosts'Connection closed by foreign host.

Do Flush host:

[root@sj ~]# mysqladmin -uroot -ppassword flush-hosts

Lets check " show slave status\G;" again.

Done.