White-Rodgers 1F79-111 Thermostat

My AC stopped working right about the time I didn’t need it anymore last year. The cool weather had begun! Then sometime near the end of the cold weather season, my heater stopped working. I then turned off the therm. altogether. For almost a month the word “LO” appeared. Now only the high temp. appears. Is the problem the battery?

Please see troubleshooting section at the manual link below. 

Manual