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Hello, you can pry the thermostat from its sub base that is screwed to the wall using a kitchen knife if you are gentle.
then if you see an R terminal and a C terminal use a volt meter to check for 24vac between those two, if you get 24v, replace the batteries again, clean the terminals while you have the batteries out... then if the screen is still dark, the thermostat is most likely bad, 98% chance.
IF you have no voltage between R and C, the problem is inside the furnace, a bad transformer, I will step you through a diagnosis if you have some tech skills and want to take the time.
Thanks, there is a wiring diagram inside the furnace blower service door, send me a close up photo of the diagram, very well lit, and exceedingly well focused.... I will mark it up with trouble shooting instructions and coach you from there.
Include a photo of the printed circuit card that shows the thermostat wire connections to it.
Use the paper clip icon at the right end of the tool bar or the attach files link to the left of your send button to get me the pictures.
We can go from there.
Thanks, ***** ***** diagram is great. I am marking it up with trouble shooting instructions now.
check for 230v power to the unit, that is *across* line 1 and line 1
Next do steps 1,2 and 3 below, let me know what you can discover