This article is part two of a three-part series on inspecting and servicing gas appliances. The information contained throughout this article series follows the gas furnace check sheet provided in the PDF link below. (Click on the link "Equipment Check Sheet" at the bottom of this article.) I would recommend that you make several copies of this check sheet to use on jobs as a reference source.
The thermostat is a temperature-controlled switch. It may or may not have selector switches for heating/cooling or fan operation. Some are still mechanical, but the majority are now digital. Some of the most common thermostat types are listed below: