Some issues that a gas furnace may have include insufficient production of heat, turning on or off too frequently or not turning off at all. Diagnosing the exact cause of the issue helps determine the proper resolution to the problem.
Issues with a furnace that doesn't produce adequate heat may be caused by the thermostat being set too low, the thermostat not working, a tripped or blown fuse, a closed control valve or the pilot light being out. It can also be caused by a hot surface ignition problem or an intermittent pilot electronic ignition problem. The issue may be resolved by making sure the thermostat is set to heat mode and that it has an appropriate temperature setting. The issue may also be resolved by cleaning the thermostat's contacts, resetting a tripped breaker, replacing a blown fuse, relighting the pilot light or troubleshooting the ignition problems.
A gas furnace not producing enough heat may also be caused by a dirty air filter, dirty gas burners, gas burners that need to be adjusted or obstructed air flow to the furnace's combustion air chamber. The issue may be resolved by replacing the dirty air filter, having the gas burners adjusted by a furnace service technician or making sure that the furnace has adequate air flow to the combustion air chamber.