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Short cycling
What is going on
The furnace lights and runs for a short time, shuts off, and repeats.
Likely causes
- Dirty filter or restricted airflow tripping the limit switch
- Dirty flame sensor or weak flame signal
- Oversized furnace or thermostat placed near a supply register
- Thermostat heat anticipator / cycle rate setting
- Partially blocked venting or condensate drain
Check in this order
- Replace the filter and check that supply and return registers are open
- Measure temperature rise and compare to the rating plate
- Read the control board fault history for limit or flame codes
- Clean the flame sensor and check flame microamps
- Check thermostat location and cycle-rate settings
- Inspect venting and condensate trap
Applies to: Gas furnaces
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Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.