This kit is proprietary to the Series FHB models

or a failing transformer

If the link is open (infinite resistance)

WeatherKing

Honeywell 301572A/0767 Thermostat Key This kit is proprietary toThe Honeywell 301572A 0767 Thermostat Key is an essential field tool designed for facility managers and HVAC contractors maintaining building security. This replacement thermostat key provides authorized access to locked thermostat covers, preventing unauthorized setpoint adjustments and tampering in commercial environments. Engineered for durability, this key is the direct replacement for standard Honeywell locking covers and is technically identical