Furnace short cycling: guided troubleshooting
Gas furnace that starts, runs briefly and shuts off, over and over. Answer each question from what you see on site.
Step 1
Is the control board showing a fault code?
Read it before cycling power or opening the blower door.
Did this solve it?
See every step in this flow
- Is the control board showing a fault code? Read it before cycling power or opening the blower door.
- Yes: Look up the code
- No: When does it shut off?
- When does it shut off?
- Burners light, then go out within a few seconds: Flame sensing
- Inducer runs, burners never light, then it retries: Draft isn't being proven, or no ignition
- Burners run a few minutes, then shut off while the blower keeps running: Is the temperature rise above the range on the rating plate?
- It heats normally but satisfies the thermostat very quickly: Is the thermostat near a supply register, in the sun, or set with a very short cycle rate?
- Is the temperature rise above the range on the rating plate? Measure supply minus return air temperature with the burners on.
- Yes, too high: Overheating on low airflow
- No, within range: Limit opening at normal temperatures
- Flames are disturbed or rolling out when the blower starts: Stop: possible heat exchanger or venting problem
- Is the thermostat near a supply register, in the sun, or set with a very short cycle rate?
- Yes: Thermostat location or settings
- No: Likely oversized
Where each path ends
Look up the code
The code names the circuit that is opening. Look it up for the exact furnace series.
Flame sensing
Lighting and dropping out in seconds points at the flame sense circuit: a dirty sensor, a poor burner ground, or flame not reaching the sensor.
Draft isn't being proven, or no ignition
Check the pressure switch, its hose and port, the vent and intake, and the inducer. If the switch closes, move on to the igniter and gas valve.
Overheating on low airflow
The high limit is doing its job. Check the filter, blower speed and wheel, evaporator coil, closed registers and duct restrictions. Never jumper a limit.
Limit opening at normal temperatures
Check gas input against the rating plate (an overfired furnace runs hot at the limit), then the limit switch itself and its wiring.
Stop: possible heat exchanger or venting problem
Flames that move or roll out when the blower starts need a heat exchanger and vent inspection before the furnace is put back in service.
Thermostat location or settings
A thermostat that is warmed by a register or sunlight ends the call early. Check its location, the cycle rate or heat anticipator setting, and for a loose W connection.
Likely oversized
A furnace much larger than the heat loss satisfies the thermostat in a few minutes. Confirm with a load calculation; staging or a lower firing rate can help where the equipment allows it.