Furnace turns off before temperature is reached, fixed by unplugging and replugging thermostat
A professional HVAC guy would have a spare tstat and swap it for testing if you were bent on keeping the old thermostat.  Unfortunately, while for a century furnace thermostats were as reliable as a rock, and nobody would ever suspect them, furnace thermostat makers have discovered if you build it just expensive enough to last 5 years, when it breaks people will just accept it because so much other electronics these days has designed-in short lifespans.  Then they will buy another.