Ihave carrier furnace 58msa code 12 flashes any solution please?

Look inside the fan compartment door for a list of diagnostic codes. The list I have doesn't define a 1-2 code. See if it's defined on your list and post it here if it is.

Code 12: Blower runs for 90 sec. If power is restored during a call for heat.
Code 21: Stuck gas valve relay on control board or miswired to gas valve circuit. It will not auto reset.
Kill power going to the furnace and then back on, this will reset the control board.
What happens? I hope that what little info I gave helps.