Goodman heater model GMT090-3B problem ignator staying on not droping out after burner comes on. Have changed board ,checked polarity cleaned flame sensor. Any suggestions.

The igniter is controlled by the circuit board and the flame sensor sends a signal to the board after the flame is established so the igniter should be off after the furnace is running.

The only thing I could see it being is a bad board but that would be strange to have two boards doing the same thing especially if the one you purchased was a known new one. I am sure you checked all the wiring also since you seem to know how to change the board. Good luck.