A furnace gas valve may be broken if the furnace does not produce heat. BobVila.com recommends ruling out other possible causes of a non-heating furnace before testing the gas valve.
If all setting switches on the furnace are in the right place, the filter is clean and the unit has power, then the problem may lie with the gas valve. One way to tell if a faulty gas valve is causing the furnace to malfunction is to check other gas appliances in the home that are connected to the same gas line. If the other appliances are malfunctioning as well, the probably is likely to be with the gas valve or line.