Error: 40 - Could not open a connection to SQL Server

Question

Error: 40 - Could not open a connection to SQL Server

Moorthy, Jayachandran 0

We are encountering an issue while importing data from a data warehouse hosted on SQL Server into Power BI. The connection is established successfully, and the import begins as expected. However, after running for some time, the process fails with the following error:

“Error 40: Could not open a connection to SQL Server.”

This error seems to indicate a connectivity problem, but what makes it unusual is that it occurs after the import has already started, not at the initial connection stage.

Has anyone experienced this type of error when working with Power BI and SQL Server? Could this be related to network timeouts, firewall restrictions, or SQL Server settings? Are there specific configurations or best practices to prevent Error 40 during large data imports?

Any insights or recommendations would be greatly appreciated.

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Answer 1

Erland Sommarskog 133.6K MVP Volunteer Moderator

Is this deterministic, or do you get the error at random places?

If the error happens in random places, this suggests transient network disruptions. If Power BI in the cloud and SQL Server on-prem this is not unlikely at all. The remedy is to implement retries, so that if you get an error, you try again rather than raising the error.

If the error occurs at one specific stage in the operations, the most likely explanation is that there is a step which has incorrect connection information. A more spectacular explanation is that an operations causes SQL Server to crash, and for some reason Power BI does notice this, but notice that SQL Server is no longer alive.

Akhil Gajavelly 1,725 Reputation points Microsoft External Staff Moderator

2026-01-06T06:11:26.1266667+00:00
Hi @Moorthy, Jayachandran,

Because the connection succeeds and the failure happens mid-import, this is not a SQL protocol or login issue.

In real Power BI imports, Error 40 almost always means the connection was dropped during a long-running query.

What reliably fixes this:

Increase Command Timeout in Power BI (Advanced Options).

Avoid loading everything in one go filter by date / use incremental refresh.

If using Power BI Service → on-prem SQL Server, verify the gateway and firewall/VPN idle timeout settings.

If the error happens at the same step every time, run that query in SSMS and check SQL Server ERRORLOG at that time.

Once the load is broken into smaller chunks or incremental refresh is used, this error typically disappears.

Thanks,
Akhil.
Akhil Gajavelly 1,725 Reputation points Microsoft External Staff Moderator

2026-04-01T06:47:44.6566667+00:00

Hi @Moorthy, Jayachandran,

Just following up to see if you had a chance to try the suggested steps. Were you able to adjust the command timeout or test the query in smaller chunks/incremental refresh?

If the issue is still occurring, please let us know whether it fails at the same step and share any relevant error details from SQL Server logs or the Power BI side we’ll help you narrow it down further.

Thanks,
Akhil.

Answer 2

The error message "Error 40: Could not open a connection to SQL Server" typically indicates a connectivity issue between your application (in this case, Power BI) and the SQL Server. Since you mentioned that the import process starts successfully but fails after some time, this could suggest several potential causes:

Network Timeouts: If the import process takes a long time, it may exceed the timeout settings for the connection. You can check and adjust the timeout settings in Power BI or the SQL Server connection string to ensure they are sufficient for your data import needs.
Firewall Restrictions: Ensure that there are no firewall rules that might be interrupting the connection after it has been established. Sometimes, firewalls can terminate long-running connections.
SQL Server Settings: Verify that SQL Server is configured to allow remote connections and that the SQL Server Browser service is running, especially if you are using named instances. If the SQL Server is set to allow only a limited number of concurrent connections, this could also lead to issues during large data imports.
Resource Limitations: Check the SQL Server for any resource constraints (CPU, memory, etc.) that might cause it to drop connections under load.
Connection Pooling: If you're using connection pooling, ensure that the pool settings are configured correctly to handle the expected load during data imports.

To prevent this error during large data imports, consider the following best practices:

Increase the connection timeout settings in Power BI.
Ensure that the SQL Server can handle the expected load and has sufficient resources available.
Monitor the network for any interruptions or issues that could affect long-running connections.
Regularly check the SQL Server logs for any error messages that might provide additional insights into the issue.

By addressing these areas, you should be able to mitigate the occurrence of Error 40 during your data import processes.

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Error: 40 - Could not open a connection to SQL Server

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