Each major quality status has an additional substatus that describes the quality of the value in more detail. The following tables describe the quality substatus for each major quality.
Good Quality Substatus Strings
The value is good. There are no special conditions.
The value has been overridden. Typically, this means that the device has been disconnected from the OPC server (either physically, or through software) and a manually entered value has been forced.
Uncertain Quality Substatus Strings
The server has not published a specific reason why the value is uncertain.
Last Usable Value
Whatever was writing the data value has stopped doing so. The returned value should be regarded as "stale." Note that this quality value differs from 'Bad: Last Known Value' in that the "bad" quality is associated specifically with a detectable communications error. The 'Uncertain: Last Usable Value' string is associated with the failure of some external source to "put" something into the value within an acceptable period of time. You can examine the age of the value using the TimeStamp property associated with this quality.
Sensor Not Accurate
Either the value has pegged at one of the sensor limits, or the sensor is otherwise known to be out of calibration via some form of internal diagnostics.
Engineering Units Exceeded
The returned value is outside the limits defined for this value. Note that this substatus does not imply that the value is pegged at some upper limit. The value may exceed the engineering units even further in future updates.
The value is derived from multiple sources and has less than the required number of good sources.
Bad Quality Substatus Strings
The value is bad but no specific reason is known.
There is some server-specific problem with the configuration. For example, the item in question is deleted from the running server configuration.
The input is required to be logically connected to something, but is not connected. This quality may reflect that no value is available at this time, possibly because the data source has not yet provided one.
A device failure has been detected.
A sensor failure has been detected.
Last Known Value
Communication between the device and the server has failed. However, the last known value is available. Note that the age of the last known value can be determined from the TimeStamp property.
Communication between the device and server has failed. There is no last known value available.
Out of Service
The Active state of the item or group containing the item is set to off. This quality is also used to indicate that the item is not being updated by the server for some reason.