Lennox Pulse 21 Troubleshooting

Check the control panel. The Lennox Pulse 21 comes with an electronic control panel. The panel displays an error code if there are any problems with the furnace. Check the control panel to see whether there is an error code. You will find a list of error codes in the owner's manual.

Reset the furnace if it will not start. To do this, open the main access panel and press the red “Reset” button.

Check the diagnostic LED light. If this is lit, the system may be locked out and you will need to check that there is adequate power to the unit and that the furnace is properly earthed. Turn off the power for at least three seconds and turn it back on. Wait three seconds and check the LED again. If it is still lit, Lennox suggests that you replace the GC-1 Control unit. You may need an electrician to assist you at this point. If the diagnostic LED is not lit after turning the furnace on and off, restart the furnace. If it does not restart and the LED light comes back on, you will need to replace the GC-1 unit.

Clean the filter. A dirty filter can cause the furnace to overheat. When this happens, it will shut off automatically. Depending on the year your furnace was made, the filter access panel will be near the bottom of the furnace on either the left or right hand side. On some models, the panel is clearly labeled with the word “filter.” Slide the filter out and clean it with cold water and a few drops of dishwashing liquid. Dry the filter thoroughly before re-installing it. You can also coat it with Filter Handicoater, which is available from Lennox.

Call for service. Have the furnace serviced or repaired by a trained and licensed electrician or furnace technician if the problem involves working with gas and electricity. Problems involving the gas supply include the furnace sputter starting and dying, the furnace starting fine but shutting off after about 10 seconds and the furnace shutting off before the target temperature is reached. Problems involving electricity include the fan shutting off, the furnace refusing to run and repeatedly turning on and off.