Furnaces tend to be fairly reliable, but like any appliance they are prone to a variety of problems, many of which can be avoided with proper care and maintenance. If your Lennox gas furnace won't start, doesn't provide enough heat or continuously cycles on and off, a bit of troubleshooting might just solve the problem without ever having to get a repairman involved.
Start with the basics. Your thermostat may have been accidentally turned off, or may need new batteries. If it is on, make sure it is set to "heat," and that the setpoint is above the current room temperature.
Go around your house and check all the vents to make sure they are open and are not blocked by furniture. If 60 percent or more of the vents are closed or blocked, it can make your furnace overheat and shut down.
Clean your air filters if you have not done so recently. The filters in most households need to be cleaned every few months, otherwise excessive buildup can block hot air from getting through. If your Lennox model uses disposable filters, they may need to be changed.
Check the gas burners. They may become dirty or clogged with carbon deposits, in which case the gas should be turned off and the burners may be removed and carefully cleaned. Once they are clean, make sure you reattach them securely.
Check the pilot light. If it is out, you may try relighting it. If it does not light, the pilot light housing may need to be cleaned. If the pilot will still not light after cleaning, there may be a problem with the gas flow, which will require the assistance of a professional.