I have a Carrier and the furnace and the heat will not come on. The fan does come on but the burners will not ignite. I get a red flash code of 2 short and one short. I have the same thermostat upstairs and I switched them and the same result. The upstairs works fine with both thermostats. I have a Carrier Bryant model number 310AAVD48070AAJA

Code:21 GAS HEATING LOCKOUT - Control will NOT auto reset.
Check for: - Mis-wired gas valve -Defective control (valve relay). You have to kill power going to the furnace to reset the control board.

T-Top is correct with the lockout code. I suggest you call a certified technician as you can cause serious injury to yourself or others trying to deal with this on your own. It sounds like the gas valve or control board may need replaced.