PMON process is a background process and it enables while instance is in mount/open mode and generally it’s deallocate the resources and releases the locks while any process is disconnect from database and pmon is responsible to register the service name in listener. PMON (process monitor ) is a mandatory process, and it’s responsibilities are as follows : - PMON is responsible for user recovery. - PMON release the locks. - PMON release the resources. - Checks the user connection. - Release the memory taken by users. - Restart the server process in the case of crash. - Shutdown the database in some cases (like redo group filled no space etc cases ) - Roll back. - Dynamical register to the database with the listener. - Incorrect environment parameter settings in SQL*Plus ($ORACLE_HOME, $ORACLE_SID). - Not enough CPU/RAM resources for the startup to complete. - Permission issues with the Oracle executables. - Leftover memory segments from a previous instance crash. - You can check the associated trace file for the ORA-00445 error.