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turn furnace off. Turn back on. code 12 should flash for 90 seconds. Then there should be a click and code change..or the unit will start...one or the other. Try this and tell me code after the 90 second 12.
Do the check out code wise.
The difference is if you turn power on to furnace while the thermostat is asking for heat or not. If it is it delays 90 seconds with the 12...then will go to regular operation