To start a Carrier gas furnace, first turn off the gas control switch, wait five minutes, and then flip the control switch back on. The automatic igniter should then engage and light the gas.
Turn the thermostat to its lowest possible setting before starting. Then, access the gas control switch through the furnace's door by removing its top screws. When you switch off the gas control, pay attention and smell for gas while you wait to turn it back on.
If you smell gas at any time while starting up the furnace, or if the igniter fails to light the gas after three attempts, contact a gas line furnace technician to diagnose the problem.