OPN or OPEN means that there is an open electrical circuit on an auxiliary temperature sensor input. In the wiring area, check to see if an auxiliary sensor has been connected to the thermostat. If there are no wires connected to the auxiliary sensor terminals, the SENSOR setting in the ADJUST MENU can be set to OFF to correct the problem.
If there is a sensor connected, check the wiring for loose connections or broken wires. In some cases the auxiliary temperature sensor may have failed and requires replacement. For specific information, locate the three digit product number of the thermostat, and enter it below to view the product page. Refer to the product Installation and Operation Manual that is available on the Technical tab. Once the issue has been corrected, it is recommended to change the thermostat to the User access level.