On Monday 3rd of August 2009 the Mission Operation Center (MOC) reported that Herschel-HIFI was found in an undocumented state since 22:43Z on August 2nd during otherwise nominal execution of a red-shifted C+ observation in band 7b. The instrument no longer responded to HK requests and commands for the LO subsystem and communication between the Instrument Control Unit (ICU) and the Local Oscillator Control Unit (LCU) had clearly been lost. At the same time the HEB mixer in band 7 had changed

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... m a nominally pumped to a completely under-pumped state and the primary power consumption of the LCU had dropped from a nominal current of 2.5A to 0.36A only. In this paper we present the results of the HIFI OD-81 anomaly investigation from early symptoms to a fully consistent failure scenario offering an explanation for what most likely had happened in space. This paper summarizes the key findings obtained from a detailed telemetry analysis and timing reconstruction, electrical circuit simulations, laboratory tests on a representative hardware model and component level tests, as well as software code analysis and simulation. We conclude by a description of the identified recovery solution and the implementation of risk mitigation measures protecting the instrument during future operations.