Furnace short cycling (starts and stops every few minutes)

What to do, in order

1. Replace the filter — the most common cause. Restricted airflow trips the high-limit switch.
2. Check that supply registers aren't all closed (closing too many chokes airflow same as a dirty filter).
3. Listen at the thermostat: if it clicks the system on/off erratically, or short cycles happen even with good airflow, suspect the thermostat (loose wiring or failing anticipator).
4. Feel the supply air: weak airflow with a clean filter = blower motor or duct issue — tech.
5. Same-day-repeat pattern after the unit's been off for months: could be a cracked flame sensor or dirty burners — annual-service territory.

Red flags — stop and call a pro

• Burner flames that are yellow/orange and wavy instead of blue — possible cracked heat exchanger / CO risk: get it checked professionally immediately
• Burning plastic smell with cycling — shut it down at the breaker
• Furnace cycling with a CO alarm event — leave the house, then call the fire department's non-emergency line for guidance

Prevention

Filter discipline, annual fall tune-up (burner cleaning + limit check), and never fully close more than 20% of supply registers.

This guide is maintained by the ee.ltd editorial team — reviewed for 2026. Browse all home repair guides or compare hvac costs.

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What this repair typically costs

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