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Universal Air Systems

Troubleshooting

Furnace Short-Cycling

Quick answer

Furnace turning on and off every 2–4 minutes is short-cycling. Most common causes: clogged filter, oversized unit, or a flame sensor that needs cleaning.

Common causes

  • Clogged filter

    easy

    Restricted airflow → overheats → safety trip → restart. Check first, fix free.

  • Bad flame sensor

    moderate

    Sensor can't confirm flame → gas valve closes → restart cycle.

  • Oversized furnace

    moderate

    Fires hard, hits setpoint in 3 minutes, shuts off. Wrong sizing on original install.

  • Bad limit or pressure switch

    moderate

    Either safety tripping prematurely → cycle starts over.

  • Thermostat in a bad spot

    easy

    Near a supply register or heat source = false reading, premature shutoff.

  • Cracked heat exchanger

    urgent

    Combustion air leaks → flame fluctuates → safety shuts down. Replace the unit.

DIY first

  • 1 Replace the filter. Wait 24 hours and watch.
  • 2 Move the thermostat or relocate any nearby heat source (lamp, electronics).
  • 3 Note the cycle pattern (run-time vs off-time) and call us with that info.

Call us if:

  • Filter swap didn't help
  • Cycles are getting shorter over days/weeks
  • Any signs of CO, soot, or yellow flame

Related service

Furnace Repair

Other symptoms in this category

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