Enterprise Manager is not able to connect to the Database instance

Answer:  The "Enterprise Manager is not able to connect to the Database instance" error is not uncommon, and there is a variety of possible solutions.

First, ensure that your environmental setting are correct: ($ORACLE_HOME, $ORACLE_SID)

On the solaris server, check your environmental setup file (.bash_profile, .bash_login, .profile, or .cshrc).

On the OEM PC, check the registry.

Check the following:

HKEY_LOCAL_MACHINE\Software\Oracle - is the home registry key.

• ORA_sid_PFILE – is the path to your pfile
• ORA_sid_AUTOSTART – tells whether the sid should start when the service does.  Generally this should be TRUE
• ORA_sid_SHUTDOWN – tells whether to shutdown when the service stops. Again, this should be TRUE.
• ORA_sid_SHUTDOWN_TYPE – How to shutdown by default when the service stops. IMMEDIATE works here.
• ORACLE_HOME – path to Oracle Home
•  NLS_LANG – nls language setting for the server

Try the following:

Bounce the OEM console using the following:

To drop old Database console

$ emca -deconfig dbcontrol db

To recreateDatabase console

$ emca -config dbcontrol db

or:

$emca -config dbcontrol db -repos recreate

Also check to see if any of the following might help:

• Bounce the OEM agent and double-check that the OEM agent is running.
  Click here for a simple script to ensure that an OEM agent is running:
  
• Ensure that the process named OracleDBConsoleyourInstanceName is running.
• Try connecting to the instance via SQLPLUS and verify that your listener.ora and tnsnanes.ora are correct.
• Follow the link to get the steps for troubleshooting connectivity.
• Regional and timezone settings: There are known issues with time zone and the "regional" setting in your control panel, see MOSC for details
• PC virus & security tools:  Make sure that you do not have anything like Norton or ZoneAlarm installed.
• Loopback adapter: Try uninstalling/reinstalling the loopback adapter, and replace the HOSTNAME with the IP address of the server.
• Verify passwords: Check that the DBSNMP, SYSMAN, and SYS passwords match the OEM configuration file. If they do not match, change the OEM config file and bounce controldb and console:
  
  Drop old OEM config files:
  emca -deconfig dbcontrol db
  
  Create new OEM config files:
  emca -config dbcontrol db
  
  or
  emca -config dbcontrol db -repos recreate

For this message:  "Enterprise Manager is not able to connect to the Database instance." there are also notes on MOSC:

MOSC note: 315299.1
MOSC note: 312308.1
MOSC note: 264067.1
MOSC note: 273758.1
MOSC note: 338398.1
MOSC note: 309687.1

Also, you can try the following for getting Enterprise Manager connected to the database instance:

In EM Database Control, try to change the monitoring configuration of the database. Make sure the dbsnmp username in the monitoring configuration page has the correct password as in the database. After this is changed, restart database control.

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