Furnace blowing cold air: causes and quick fixes

What to do, in order

1. Thermostat check first: HEAT mode, fan AUTO (ON forces continuous cold-air circulation between cycles).
2. Watch the furnace start: if you hear the inducer fan, then clicking with no flame, the igniter or flame sensor is the prime suspect.
3. Find the small reset/limit switch — some units lock out after 3 failed ignition attempts; power-cycle at the breaker for 60 seconds to reset.
4. Check the filter — an over-limit condition shuts burners while the blower keeps running, blowing cold air.
5. Gas valve on? Verify it's parallel to the pipe (open). If you smell gas at ANY point: leave, don't flip switches, call the gas utility from outside.
6. If burners light for 5–10 seconds then die: dirty flame sensor. Needs cleaning or replacement — a common $80–$200 visit.

Red flags — stop and call a pro

• Gas smell — evacuate and call the gas utility, not an HVAC tech
• Soot or scorch marks around the burner compartment — shut it down and call
• Yellow instead of blue flame (natural gas) — combustion problem, carbon monoxide risk
• Cold air plus a CO alarm — get out, then call 911

Prevention

Annual fall tune-up with flame-sensor cleaning and combustion check; keep the area around the furnace clear; test CO detectors when the season starts.

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

Related guides

• AC running but not cooling: 6 checks before you call
• Furnace short cycling (starts and stops every few minutes)
• AC evaporator coil frozen: why it happens and the thaw protocol
• Thermostat not working: dead screen, wrong temps, no response

What this repair typically costs

• AC repair cost guide
• AC replacement cost guide
• Furnace replacement cost guide
• Heat pump installation cost guide