My Bryant 350MAV furnace is not working properly. While the blower is first running it flashes a code 12. Then the light goes on solid and the burner never ignites. If I hold the manual rollout reset button for 20 seconds the ignitor starts to glow and the burner ignites.

What needs to be replaced?

What needs to be replaced will be determined buy what is actually causing the rollout switch to activate. You need to find the exact problem before you can effect the cure.

OR it could be as easy as the rollout switch is defective or out of range.

How old is this unit? Post serial#. Roll-out switch problems are usually the result of heat exchanger problems. Some of these had faulty secondary HX.