Why do I get 80040200 / 80040514 / 800A0E7A errors?

If you are getting one of the following errors: 
 

ADO could not find the specified provider.    Provider cannot be found. It may not be properly installed.    Data source name not found and no default driver specified.    Unspecified runtime error.    Library not registered.    Class not registered.

 
First things first: check your connection string. This error can often happen if you have a typo in your provider or driver details. 
 
If you are sure that your connection string is accurate, then follow these directions, depending on the database platform in use: 
 
SQL Server 
 
Make sure you have the most recent version of MDAC installed (MDAC Download Page). 
 
Access 
 
This is often caused by attempting to connect to an Access database using OLE-DB (e.g. Provider=Microsoft.Jet.OLEDB.4.0;) without having JET components installed. A major source of this problem is that Microsoft stopped shipping the JET files with MDAC, starting at 2.6 (see KB #271908), so many machines set up starting at that baseline or later do not have the necessary JET runtime files.  
 
See Article #2342 for information on getting the most recent version of the JET provider. 
 
Other problems with Access include fat-fingering the connection string, or mixing up Access and JET parameters, e.g: 
 

Driver={Micrsoft Axess Diver (*.mdb)};DBQ=c:\file.mdb  (several misspelled words)    Driver={Microsoft Access Driver (*.mdb)};Data Source=c:\file.mdb  (DBQ is the parameter for database location with the Access driver)    Provider=Microsoft.Jet.OLEDB.4.0;DBQ=c:\file.mdb  (Data Source is the parameter for database location with the JET provider)

 
See Article #2126 for sample Access connection strings to help model your own. 
 
Any other vendor 
 
Make sure you have the most recent version of MDAC installed (MDAC Download Page), and check the vendors' web site and make sure your server has the most recent version of their ODBC driver or OLE-DB provider. 
 

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Under certain scenarios, connection pooling may cause the following error: 
 

Microsoft OLE DB Provider for ODBC Drivers error '80040200'  [Microsoft][ODBC Driver Manager] Only SQL_DRIVER_NOPROMPT is allowed when connection pooling is enabled

 
To make this error go away, you can override the prompt property of the connection object in the following manner: 
 

<%      const adPromptNever = 4      set conn = CreateObject("ADODB.Connection")      conn.Properties("Prompt") = adPromptNever      conn.open <connection string>      ...  %>

 
For more information, see KB #167745, KB #200886 and KB #253114. 
 

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If you are using any of the fancy wizards that come with Visual Studio 6.0, you may have this error message: 
 

Supplied provider is different from the one already in use

 
My advice: don't use DataGrid, DataEnvironment, ADODC, DTC or any other 'automatic' / helper control, or at least don't try to combine provider=MS.DataShape with data provider=Microsoft.Jet.OLEDB.4.0. 
 

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And finally, if you are using CDO / CDONTS with (or trying to replicate) Outlook Web Access, you may get one of the following: 
 

MAPI_E_END_OF_SESSION    CdoE_END_OF_SESSION

 
You may also see Event ID 4096 in the event log. 
 
If you see any of the above, please refer to KB #297969 for information and potential resolution(s).