Troubleshoot gas furnaces by checking for common issues such as incorrectly set or malfunctioning thermostats, clogged shutoff valves, dirty filters and clogged pilot openings. By checking for these issues initially, more serious problems can hopefully be ruled out.
Start by ensuring that the thermostat is set correctly. Make sure that the thermostat is switched on, and the set point is above room temperature. If the furnace’s fan is not running, place it into "on" mode.
If the gas furnace is generating insufficient heat, check whether the blower and the filter are blocked or too dirty. A blocked blower or a dirty filter can also cause erratic heating. Clean and lubricate a clogged blower, and replace a dirty filter with a new one. If the problem is caused by an incorrectly set thermostat, adjust its settings to resolve the problem.
If the furnace’s pilot does not light, check if the pilot opening is blocked or the gas valve is closed. Clean the pilot opening and confirm that the gas valve to the furnace is fully open. If the pilot still does not stay lit, adjust the pilot light so that the flame is about 2 inches long. If the problem persists, tighten the thermocouple, or replace it if it is faulty.