The furnace thermostat is the control device that determines when and how much heat will be produced by the furnace. This tutorial describes how to troubleshoot the low voltage thermostat often made by Honeywell, White-Rodgers, Lux, Robertshaw and others. This type of thermostat runs off of a transformer reducing the incoming line voltage from 120 volts down to between 12 and 24 volts, depending on your furnace.
See How to Troubleshoot a Gas Furnace for information on possible furnace problems, causes and repairs.
But now let's take a look at troubleshooting problems that could be caused by a faulty thermostat and how to make the repairs.