After SQL was removed from Failover Cluster Manager I was unable to uninstall it. The following error was logged in the application logs:

Product: Microsoft SQL Server 2008 Database Engine Services — Error 25012. There was an error attempting to remove the configuration of the product which prevents any other action from occuring. The current configuration of the product is being cancelled as a result.

To correct this I needed to follow the steps below to tell SQL it was no longer in a cluster. Then I was able to uninstall normally.

1. Open Registry Editor (Start -> run -> type in ‘regedit’)

2. Navigate to the following key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10.MSSQLSERVER\ClusterState\SQL_Engine_Core_Inst

3. Change the value of this “SQL_Engine_Core_Inst” key from 1 to 0.

4. Uninstall as normal through Programs and Features

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