This test will verify that low-voltage R is truly the hot wire. IF this test produces roughly 145 Vac then this means that the transformer low voltage is out of Phase. *** The easiest way to correct this is to swap SEC-1 & SEC-2 on the board. (this is commonly seen when people replace transformers on furnaces)

Third, if the high voltage and low voltages have been checked for proper phasing and they are correct, and the board is still showing a rapid flash, at this point it is recommended to replace the furnace control board.

Lastly, If everything above checks good and the board is changed and the unit produces a rapid flash then you could (temporarily) try wiring the furnace to an extension cord and plugging it into a receptacle within the home. After ensuring the L1 and N are correct once more, if this corrects the rapid flash then the issue is with the breaker feeding the furnace.

There will/can always be a “one in a million” situation but for the vast majority this guide will assist in finding the reason for the furnace rapid flash.