Thermostat is reading crazy high temps when it is really 67. Furnace won't deliver heat. Fan works. Is it the thermostat (batteries replaced already) or the furnace?
Two long for a comment Yes thermostats do go bad. The old double mercury thermostats are great for simplicity and no batteries needed,
Yes digital thermostats do fail. Just like all electronics they have a life and if in an electrical noisy area the life will be shortened. I have only seen a few thermostats that were actually integrated as part of the heating / cooling system. Depending on the features your system has you will need a common wire a call for heat and a call for cooling, fan only, different fan speeds / heating levels . You can install the old thermostat and although it won’t cost anything today loosing some of the features of a modern digital may be worth the expense. Most only have cooling/ heat a fan only setting and off.