This code system http://hl7.org/fhir/metric-operational-status defines the following codes:

CodeDisplayDefinition
onOnThe DeviceMetric is operating and will generate DeviceObservations.
offOffThe DeviceMetric is not operating.
standbyStandbyThe DeviceMetric is operating, but will not generate any DeviceObservations.
entered-in-errorEntered In ErrorThe DeviceMetric was entered in error.