ORA-12541: TNS:no listener

The client reached the host, but nothing is listening on that port. The listener is stopped, or the port in your connection is wrong. Start the listener or correct the port.

What ORA-12541 means

The listener is the process that accepts new connections and passes them to the database. ORA-12541 means the host answered and refused the connection on that port. So the network path works, which rules out most firewall problems. Newer releases word the message as Cannot connect. No listener at host … port ….

Common causes of ORA-12541

The listener is not running, for example after a server restart.
A wrong port in the connection or in tnsnames.ora.
A wrong host: the server you reached is not the database server.
The listener listens on another address, such as localhost only.

How to fix ORA-12541

On the server, check it: lsnrctl status.
Start it if needed: lsnrctl start. On Windows, check the listener service too.
Compare the port and address with listener.ora.
Once it is running, the database should register within a minute; ALTER SYSTEM REGISTER speeds this up.

ORA-12541 example

On the database server:

lsnrctl status
lsnrctl start

sqlplus / as sysdba
SQL> ALTER SYSTEM REGISTER;

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Oracle documentation for ORA-12541. Oracle is a registered trademark of Oracle Corporation. SQLlightspeed is not affiliated with Oracle.