What could cause a furnace to cycle off shortly after starting up, with the blower not turning on?

A defective thermocouple can cause a furnace to cycle off shortly after startup because the thermocouple is a safety device that detects whether the pilot light is on. If the thermocouple is malfunctioning, it may falsely signal that the pilot light is out, which prompts the furnace to shut down to prevent the buildup of unburned gas. This shutdown occurs before the blower has a chance to turn on, as the system does not reach the operational stage required for the blower to activate.

In contrast, a blockage in the air intake would typically lead to overheating and a safety shutoff later in the cycle rather than an immediate shutdown. A faulty thermostat setting might not trigger an immediate shutdown but would instead lead to inconsistent heating or cycling behavior over a longer period. Issues with the gas supply could prevent the furnace from igniting or functioning at all, but once ignited, they would not directly lead to an immediate cycle-off unless the supply was entirely cut off during operation.

Overall, the thermocouple plays a direct role in the safety operation of the furnace, making it the most likely cause of a prompt shutdown without the blower engaging.