The electrode both creates a spark to light the burner, and senses that the burner is lit. If either task fails, it will cause the symptoms that you are experiencing, and the run time will become shorter and shorter. The fact that it runs for 5 hours, suggests that the problem is probably not with the control board. Also, the electrode often fails after 4 or 5 years of use.
The electrodes are not very expensive. and are not difficult to replace.
Retired electronics engineer. Avid paddler & birder.
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