Short cycling is the term for a furnace that starts, runs briefly, and shuts off again before it has finished a normal heating run. It is worth noticing because it changes how the house warms, how long the blower runs, and how much work the system is doing. It is also something you can observe without taking the cabinet apart.
What you can observe
Stand near the furnace while it is calling for heat. Listen for the burner and the blower. If the burner lights, runs for a short time, and goes out, and then starts again within a few minutes, that pattern is short cycling. Notice whether the room warms unevenly, with a hot spot near the supply register and a cool spot farther away. Notice whether the thermostat reads higher or lower than the room seems to be.
Check the easy things first
Look at the air filter and the return grille. A dirty filter or a blocked return can make the heat exchanger run hot and trip its limit switch. Check that the thermostat is not sitting in a sunny window, near a lamp, or on a wall that gets heat from another room, since any of these can trigger early shutoff. Write down what you find, the date, and whether the pattern changed after you made a change.
Know when to stop and call
If you smell gas, leave the house, do not use switches or flames, and call the gas utility or emergency number from outside. For any other short cycling, call a qualified heating technician and describe what you observed. Ask the technician to check the flame, the limit switch, the vent, and the airflow, and to write down the findings. Keep the record in the service log so the next visit starts from what you already saw.
Plan for the unit's age
A furnace that is short cycling is still a furnace with a service life, and the age of the unit is one of the planning inputs. Compare the age you have on record with the typical service life on the gas furnace page, and use the fall furnace walk-through to set up the checks before the heating season begins.