There can be several reasons why your Honeywell thermostat ProSeries is resetting to 85 degrees by itself. Some potential causes and troubleshooting steps to resolve the issue are as follows:
Power Supply Issues: Check if there are any power supply problems that might be causing the thermostat to reset. Make sure the thermostat is properly connected to a stable power source. Loose or faulty wiring can lead to intermittent power supply, causing the thermostat to reset.
Firmware or Software Glitch: Like any electronic device, thermostats can experience software glitches that may lead to unexpected behavior. Check if there are any firmware updates available for your thermostat model. Updating the firmware may resolve any known bugs or issues.
Electrical Interference: External electrical interference, such as from nearby appliances or electronics, can disrupt the thermostat's functioning. Ensure that the thermostat is placed away from sources of interference.
Battery Issues: If your thermostat uses batteries, make sure they are not depleted or improperly inserted. Low or dead batteries could cause the thermostat to reset or behave erratically.
Temperature Sensor Problems: The thermostat's temperature sensor might be malfunctioning or getting inaccurate readings. This could cause the thermostat to behave unexpectedly, such as resetting to a default temperature.
Wiring Problems: Faulty or loose wiring connections can lead to sporadic issues with the thermostat's operation. Check the wiring and connections to ensure they are secure and properly connected.
Thermostat Settings: Review the thermostat's settings to see if there are any pre-programmed schedules or temperature settings that could be triggering the reset. Additionally, check if there are any "lock" features that might restrict temperature adjustments.
HVAC System Issues: There might be an issue with your heating, ventilation, and air conditioning (HVAC) system itself, rather than the thermostat. Problems with the HVAC system could lead to abnormal behavior in the thermostat.
If you have checked the above points and the issue persists, it might be a good idea to contact Honeywell customer support or have a professional HVAC technician inspect both the thermostat and the HVAC system to diagnose and resolve the problem accurately.