The digital display on your Honeywell RTH8500 thermostat could become locked for a number of reasons. Fortunately, this problem is usually easy to solve. By following the steps below, you should be able to unlock the display and restore the thermostat to its normal state.
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Remove the thermostat from its base and replace the batteries. To remove the thermostat, grasp it at the edges and pull toward you. The batteries are on the back of the thermostat housing. If this doesn’t unlock the touch screen, continue to the next step.
Check that the furnace’s fuse is not blown or that the circuit breaker is not tripped. Replace a blown fuse or reset a tripped circuit breaker, and try the thermostat again.
Press the “Screen” button on the touch screen, then wait for 30 seconds and press “Done.” In many cases, this will unlock the screen and the thermostat will be ready for normal use.
Wait five minutes for the compressor timer to reset if “Wait” is displayed on the screen. Then operate the system normally.