Return code when server is down


Could someone kindly confirm what the server response is when the Companies House service is down and/or unavailable?

From what I understand the CH API does not return the standard 503 response? We are writing a series of integration tests and would like to test the scenario where the CH API server is down or faulty and confirm what the response code and status should be (as I understand the 404 status is also used for a company that is not found).

Thank you in advance.