Furnace Code 12

Also known as code 121 on 2023+ furnaces

When the logic center reboots (while the furnace has a 24vac call on W) it does not know if the burners were burning prior to the voltage drop. So, to be safe, the logic will run the blower only for 90 seconds instead of firing the burners and entering the blower off delay (as a normal call for heat would do). THIS is what a code 12 is (blower on after powering up). Without this safety, in the event a blower delay was initiated while the heat exchanger was already hot, then the chances of burning a hole in the heat exchanger would be high.

Since the code 12 is only initiated when the logic sees 24vac on W at the same time power is applied to furnace logic, this will allow you to bypass the 90-second delays that slow you down.

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