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The digital thermostat lights up with "HEAT ON" but it takes the furnace up to an hour to kick start. Once it does, it puts out heat. We set it to 69 here in Michigan with the outside temp at Zero. When it reaches 66, sometimes it just quits and doesn't come back on for a long time.

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Your furnace tries to ignite five times. If it fails, it goes into lockout for 60 minutes. Once the lockout period is up, it tries five more times.

From your description, it sounds like it's failing the first set of ignitions. This could be a problem with the gas flow, a bad igniter, a misaligned igniter, a dirty or bad flame sensor, etc.

The best way to start diagnosing the problem, is to observe the ignition sequence. Set the thermostat to heat, and turn the set point down so the thermostat calls for heat. Then go to the furnace, and watch/listen to what's happening. The normal ignition sequence will be something like...

  1. Inducer fan starts.
  2. 15 seconds later the hot surface igniter heats up.
  3. After 20 seconds the gas valve opens.
  4. The gas ignites, and the flame sensor "proves" the flame.
  5. After a delay, the blower will start.
  6. The furnace runs until the thermostat is satisfied.

If step 4 fails, the furnace tries four more times to ignite. If it still fails, it goes into the 60 minute lockout.

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  • Thanks Tester101, your response put me on the right track and confirmed my thoughts. Took out the Flame Sensor, cleaned it out with an emery cloth and clean rag and Mr. Lennox kicked on and has been working great for 24 hours now. We are experiencing below zero weather and waiting 1-2 hours for the heat to kick was starting to get bad. – Ike_C Feb 19 '15 at 4:23
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From your description you either have a faulty thermostat or an intermittent issue with the furnace.

The next time this issue happens go down to the furnace and check the diagnostic lights. They are located behind a circular sight glass in the lower door of the furnace. You will have to get right down onto your hands and knees and look directly into the sight glass to see them. Refer to this chart to see if the unit is showing any fault codes, or if it is waiting for a call for heat.

G40UH error codes

When you have the thermostat calling for heat and the unit is not coming on check to see if the furnace diagnostic led's are simultaneously slow flashing. This means that the furnace is waiting for a call for heat and is in normal operation mode. If this is the cause then you will most likely have an intermittent issue with the thermostat. If any other error code comes up then the chart will direct you to the source of the issue.

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  • Thanks Mnc123, your response put me on the right track and confirmed my thoughts. Took out the Flame Sensor, cleaned it out with an emery cloth and clean rag and Mr. Lennox kicked on and has been working great for 24 hours now. We are experiencing below zero weather and waiting 1-2 hours for the heat to kick was starting to get bad. – Ike_C Feb 19 '15 at 4:23

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