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Find the flame sensor or 'ionization rod' on the wiring diagram, then follow the wire to the flame sensor mount on the fire box, remove it, and clean the long sensor rod off with a scotch brite pot scrubber pad, or *plumbers sand cloth.
If that fails, take a clear, well focused photo of the wiring diagram glued to the inside of the service panel, close up and from straight on, and attach it here using the paper clip icon at the right end of the tool bar, or the '+attach files' link to left of your send button.
I will mark it up with trouble shooting instructions.
We can go from there.