The things required before beginning with the troubleshooting of Lennox furnaces include a vacuum with a brush attachment, soft brush and a clean cloth. First, check whether there is some power to the Lennox furnace.
If there is absolutely no power to the furnace, plug the furnace into a well grounded power outlet and reset or replace the fuse or circuit breaker, as per the requirement. If there is a supply of power and heat to the furnace, but it is still not functioning, switch the furnace thermostat at a temperature that is higher than the ambient temperature. To obtain the warmer temperature, set the fan speed to auto and the thermostat setting to heat.
In case the furnace does not provide the expected heating, clean the filter. Disconnect the power supply to the furnace and remove the filter out after opening the access panel. Remove the dirt using the vacuum. After cleaning the filter well, place the filter back in place and close the access panel.
Next, if the furnace is noisy clean the fan and the blower wheel by using the soft brush. If the air flow is not as expected or if the operation is very noisy, clean the grills and vents on the furnace using the clean cloth.
Even after troubleshooting in the above manner, if the issue still continues, contact the Lennox service centre.