Troubleshooting Alternating Lights on a Lennox G40UH

The Lennox G40UH is a whole-home gas furnace with a combustion air pressure switch and Lennox’s DuraLock heat exchanger assembly. The furnace has a control panel where the automatic ignition pilot light can be viewed. There is a light on the control panel that will blink rapidly if there is a problem with the furnace that requires attention. Alternating lights on the control panel indicate that the burners failed to ignite, which requires some troubleshooting.

Vacuum cleaner

Instructions

- 1
  Disconnect power to the Lennox furnace.
- 2
  Slide the air filter out of the main access panel and clean with a vacuum cleaner if clogged with lint or dirt. Return the filter to the furnace and close the access panel. Reconnect power to the furnace.
- 3
  Replace or reset any necessary circuit breakers or fuses and make sure the gas valve attached to the furnace is turned on (counterclockwise).
- 4
  Look at the manual main shut-off valve on the furnace and make sure the valve is open.
- 5
  Set the room thermostat to a temperature above the current ambient temperature. The furnace should automatically ignite within a few minutes.

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