Most common causes
- Clogged air filter. Low airflow makes the furnace overheat, and the high limit switch shuts the burners off. It cools down, restarts, and repeats.
- Dirty flame sensor. The furnace lights, the sensor cannot confirm the flame, and the gas shuts off within seconds. It may try a few times and then lock out.
- Blocked vent or intake pipe. On high efficiency furnaces, leaves, nests, or debris in the PVC pipes outside trip the pressure switch.
- Thermostat placement or settings. A thermostat near a supply vent, a heat source, or in a drafty hall reaches the setpoint early and stops the cycle.
- Oversized furnace. A furnace that is too big for the house heats the space too fast and shuts off before running a full cycle.
- Cracked heat exchanger or failing parts. Some short cycling comes from failing limit switches, a weak inducer motor, or heat exchanger problems. These need a technician.
What you can safely check
- Replace the furnace filter.
- Open all supply vents and clear return grilles.
- Check that the thermostat is not near a vent, lamp, or sunny window, and replace its batteries.
- If you have a high efficiency furnace with white PVC pipes going outside, check that the pipe ends are not blocked by leaves, nests, or plants. Do not take pipes apart.
- Note the blinking code through the furnace door window before you turn the power off. That helps the technician.
- Make sure CO alarms in the house are working.
Call a technician if
- It keeps short cycling with a new filter
- The status light shows a limit, pressure switch, or flame code
- You smell burning, see soot, or the flame looks yellow and lazy instead of blue
- A CO alarm sounds (leave the house and call 911)
- The furnace locks out and will not restart
What affects the repair cost
A filter, flame sensor cleaning, or clearing a vent pipe is a simple fix. Replacing a limit switch, pressure switch, or inducer motor depends on the part and model. A cracked heat exchanger is a major repair and on older furnaces often leads to replacement, which a technician should confirm with an inspection.
