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 or a fail. If a failure occurs, the rest of the tests are not run. Some test fails that are not fatal produce a warning message, and the tool will continue to run the subsequent tests. See the details below for a brief description of each test, and possible reasons for its failure.