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Oracle Client + ORA-12557: TNS:protocol adapter not loadable

Deat experts, We migrated from Oracle 12c to Oracle 19c in the windows server 2016 last week.

The DBA did not remove the previous version of oracle client - 12c and in addition installed the latest version of Oracle 19c.

Also I can see the environment variable has below 2 entry in addition to both the client installation



When we connect to the database using external connection, we get the below error. The DB connection was working fine prior to the oracle 19c installation.

The program has ended abnormally due to an unhandled exception.
Exception Code (OCI[DBConnectOCI.cpp[192]]
ORA-12557:TNS:protocol adapter not loadable)


  1. I am not a DBA person but technically oracle has what it needs, why does it not getting connected to the DB using oracle client 19c?
  2. Is it 100% true that we need to remove the previous version of the DB Client and always have only one client version?
  3. Is the environment variable creating the issue ? or both env variable and oracle client ?


  • Sunny kichloo
    Sunny kichloo Member Posts: 2,460 Gold Trophy
    edited Apr 4, 2022 1:03PM

    Try to run below mentioned command



    In this command do confirm any environment variables using old oracle version.

    If it is just Oracle client Try to update old oracle version path with new one in environment variables like ORACLE_HOME,PATH or any others.

    Do share the output and if it is not connecting after above mentioned changes do share error message.

  • nabjoshi-Oracle
    nabjoshi-Oracle Posts: 14 Employee

    set below and try the connection.

    set oracle_home=c:\app\oracle\product\19c\dbhome_1 ( set the 19c home)

    PATH=%ORACLE_HOME%\bin; set PATH

    set TNS_ADMIN=%ORACLE_HOME%/network/admin

    Let me know if it works.

  • EdStevens
    EdStevens Member Posts: 28,778 Gold Crown

    Try to run below mentioned command


    That's a linux command. The OP is clearly on Windows. The Windows equivalent is 'set'.

    Sunny kichloo