422 blower fan issue

[maine islander]

Burner comes on and flame starts to built as normal.
Problem is fan never comes on and burner shuts off after getting hot.
I am assuming the shut down is occurring as a safety feature but don't understand why fan is not activated.
I can bypass the fan's temp relay and it will come on but the burner still shuts down.
Any ideas.

[hawkins111]

The flame sensor has lost the flame very early in the cycle. The timer on the main board has not gotten to the point of allowing the circulating fan to come on. You need to open up your burner and install a new mat and burner ring.

[maine islander]

Update:
@ Hawkins111, thanks for the reply. I had figured it might have been the sensor rod so removed it, cleaned off soot reinstalled, same problem. I then took off burner access plate and looked at area at bottom of sensor rod expecting to find more soot and carbon build up. I found a piece of the white gasket had broken off and fallen down below rod, obviously keeping flame from rod. Vacuumed out the piece and any soot in burner. Did the trick, fired back up heater fan came on and we are back in business.