The diagnostics tool is only available for remote projects and remote connections, and is run from within the Eclipse IDE: select a remote project (in COBOL Explorer) or a remote connection (in Remote Systems Explorer), then right-click and choose Diagnose Connection Problems to run the tool.
The tool runs a series of tests, which result in a pass, warn, or a fail. See the details below for a brief description of each test, and possible reasons for its failure.