Furnace Rapid Flash

This article is in reference to non-twinned furnaces. Pictures are shown of a Carrier 58STA110-13122

The manual and sticker for Carrier/Bryant/Payne furnaces show the following:

At first glance, this seems simple. Just swap the two incoming power wires, right? But the furnace has been running for years at the home with no issues and swapping the wires produces the same rapid flash situation…. What gives?

Let’s tackle how to attack this fault:

FIRST, trace the wires or read the schematic to determine where L1(black) and Neutral(white) land on the board- Pointed in green. Use a voltmeter to Verify the follow:
L1-N= 120vac
L1-Ground=120vac
N-Ground=0vac
This will verify that the ground to the equipment is good and that L1 is the HOT wire and not N. ***If L1 to Ground is 0vac and N to Ground is 120vac then simply reverse the two wires as they wire nut to the furnace power in the Junction Box highlighted in Purple.

Second, Use the Voltmeter to check AC voltage from R (on the screw terminal strip) to L1. This test SHOULD read mid-90’s, my test produced 93.4Vac shown here:
This picture is an actual reading with my meter as well as a layover of the schematic showing the terminals that I am checking.

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.