Hi, my name is XXXXX XXXXX I have been in the plumbing and plumbing repair field for 20 years. I will be glad to help you today. First check the condensate drain is clear. If condensate has backed up into the fan it could have shorted the fan. After confirming the drain is clear inspect the connector on the fan and see if there are any signs of shorting. Also if the flame sensor is dirty remove it and sand the end gently with scotch brite until shiny but not so much that you take the top layer off.
It could also need that but doubtful that would cause lockout. Flame failure is usually sensor related but a dirty heat exchanger can certainly slow it down too.
If you have further questions, I will be back in about 30. Gotta go read some bedtime stories.
I am back if you have more questions
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