What could be meant by "Open output" in this context?
You have a temperature sensor with analog voltage or current output.



Different operating modes, but one is the basic process controls 4-20mA two-wire current loop meant to be fed into a remote &lt;600Ω load resistor.  3.5mA or 23mA (NAMUR NE43) are used to indicate errors.  And 0mA indicates an open circuit or unconnected sensor.

Not the same, but relevant from 4104 Universal Signal Transmitter:


  Therefore, a continuous measurement of the outgoing current is carried out on a 4...20 mA output signal.  If the analogue output current is 0 mA (can e.g. be caused by an open output loop)....


Same thing on the voltage side.  Take 0-10V feeding ≥10kΩ resistor.  Solid 0V or 11V (+10%) indicate errors.  &lt;18V indicates an open circuit or unconnected sensor.

If the communication loop for the sensor is working properly in 0-10V mode, output will vary from 0V to 10V.  Actual range depends upon dip switches.  Solid 0V or 11V = sensor errors (upscale or downscale, broken sensor), while something >11V but &lt;18V means communications loop is open.

In current mode, an open circuit with 0mA is easy to detect on receiving side, but in voltage mode, voltage on the receiving side for an open circuit is 0V.  So the >11V but &lt;18V would be detecting the open circuit on the transmitting side to indicate an error.

In your case, you will have R5/R4 voltage divider if circuit becomes open.  Your circuit will not see 18V. 

From 3000 series Users Manual