My Lennox furnace isn't turning on. What could be the problem?

My Lennox furnace isn't turning on, what could be the problem includes several common causes from simple thermostat to failed igniter, and you should check in order before calling technician for models like ML296V, EL296V, and SLP99V. First, check thermostat set to heat and temperature set above room temperature at 75 degrees, and thermostat batteries not dead, and thermostat not showing blank display. Second, check furnace power switch on side of furnace is on, and breaker at panel not tripped, and furnace door closed properly as safety switch prevents operation if door open. Third, check air filter dirty causing high limit switch to trip and shut off furnace, replace filter every 2 to 3 months. Fourth, check gas supply valve open and other gas appliances like stove working, if gas off, furnace will not light. Fifth, listen for igniter, if furnace clicks but does not light, igniter may be failed after 5 to 8 years costing 100 to 200 dollars, Lennox igniter glows orange, if not glowing, replace. Sixth, check flame sensor dirty causing furnace to light then shut off after 5 seconds, clean sensor with fine sandpaper. Seventh, check condensate drain clogged for high efficiency furnace causing pressure switch to trip, clear drain. Eighth, check error code flashing on control board LED through small window, count flashes and look up code in manual, for example 3 flashes for pressure switch. If these checks do not fix, call technician for control board at 400 to 700 dollars.