I have a Carrier 58PAV090-14. The LED error code is flahing 12. The draft fan starts. Ignitor glows. Flame starts. Heats up. Blower starts and runs 5 seconds then all stops. I swapped the pressure switch and flame sensor with my other, identical unit that is working. No luck. Help?

It is not your pressure switch, that is the first safety control , that will bring on furnace . Not to sure if this is a in shot burner system ? Sounds like there is a temp limit tripping out . Do you have good air flow ? How old is this system , You may have a damage heater cell ? When the system is running try to find out the safety that is open up. Hopefully this will help you out.