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Have you tried cleaning the flame sensor?
OK if flame sensor is clean, make sure the ceramic does not have a crack,Also unplug the wire and make sure connection is clean both on board and at sensor,Make sure there is no breakdown anywhere in that orange wire.. Do you have a meter that will read micro amp so we can see if we are getting at least 2.8 microamps to the board from flame sensor?
Does your board have a light on it and if so does it give us an error code when burners go off such as 3 flashes then pause?
OK.First make sure equipment has a good ground.Then check the micro amps and verify flame sensor is sensing flame and sending at least 2.7 micro amps to board to tell board to keep gas valve open.If so replace board.
Below is a short video showing how to check microamps: