Ever since the cold spell has settled in here in central Pa, I have been having a problem.
Thermostat is good ole Honeywell, round, mechanical. I do not know what make or model furnace is. We bought the house last May and home inspector told us furnace looked to be around 15 yrs old. He did say the furnace looked to be in good condition. I have looked over furnace and can not find any information on it. There is one spot where there was a sticker, but sticker is gone. Furnace is gas, forced air.

The problem is the thermostat is set for 74 degrees and the furnace cuts off at 68 degrees.
The 68 degrees is the reading on lower half of thermostat. I also have a digital thermoter on my computer desk and it is reading 68 degrees. Thermostat has been accurate within a degree or two up to this last cold spell set in.

My question is: Does this sound like a furnace or thermostat problem? Should I just try replacing the thermostat first before I call for a service call?