The thermostats switches on the power, which opens the gas valve. After it warms up a limit switch, the blower relay starts the blower. I hear the blower come on, but the fan is not turning and another limit switch shuts the gas valve off. This gas valve and blower keep cycling on and off since the blower fan does not turn on and the temp is still low.
I have a Carrier 58GS series Upflow Gas-Fired Furnace.