Check all the electrical disconnect switches and make sure they are all set to "On." If these switches are in the "Off" setting, the furnace will not activate, so flip them to "On" if necessary.
Check thermostat settings to make sure there is a heating demand for the Lennox G40UH(X). Like all furnaces, the Lennox G40UH(X) only activates when its thermostat is set above room temperature. Therefore, if the thermostat is set at or below air temperature, move it at least 5 degrees Fahrenheit above room temperature and see if the furnace activates after a few minutes.
Clean or replace the furnace's air filter. If the Lennox G40UH(X)'s air filter becomes clogged or saturated with dirt and dust, the filter will limit air flow to the furnace and the furnace will not function normally. Test the Lennox G40UH(X)'s functioning after cleaning or replacing the air filter.
Verify the gas supply to the furnace by checking the gas shutoff valve that supplies the furnace and make sure it is open. If it is, contact your gas provider and make sure your gas service has not been interrupted due to non-payment, a service outage or for any other reason. If the furnace has a steady supply of gas, contact a Lennox representative for further assistance.