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This should be the manual for your unit, the XL model shown in the first column with page numbers
Tell me a lot about your technical skills and if you have a multi meter or not, so that I can coach you most effectively.
We can go from there.
Please attach a well focused, close up photo of the wiring diagram glued inside the service panel,you can use the paper clip icon at the right end of the toolbar at the top of this dialog box,Include a picture of the thermostat wire connections to the printed circuit board in the same area.
If there is no paper clip icon, look for the '+attach files' link at left of your send button.
If you can include a picture of the knob on top of the gas valve that will help as well.
So far it sounds like a bad thermocouple, or flame sensor.. or a slightly lesser chance of a bad high temperature cut out switch.