Gas furnace running but blowing cold air? Start here

Last verified: Oct 24, 2026

Disclosure: Some links on this site may be affiliate links; see our disclosure. Not affiliated with Carrier, Trane, Lennox or Rheem. Safety first: if you smell gas, don't touch switches or light anything. Leave the house and call your gas utility or emergency services from outside. Carrier says not to diagnose or repair a gas leak yourself. [1] If a carbon monoxide alarm sounds or anyone has headache, dizziness, nausea or confusion, CPSC says get outside to fresh air right away, then call 911. [6] Turn off the furnace power before you open any panel. Gas, burner and control board work is for a qualified HVAC technician.

It happens on the first cold night of the season: the thermostat clicks, the fan runs, and the vents blow cold air. Before you pay for an emergency call, run the free checks below. Carrier, Trane and Lennox all list most of them. [1][2][3]

First: is it really a problem?

A short burst of cool air at startup is normal. Carrier says some furnaces "may delay the blower for approximately 30–60 seconds after the burners ignite," and if cold air lasts "longer than a few minutes," it's time to troubleshoot. [1] Lennox says cold air at first is normal when the furnace hasn't run for a while, because the air sitting in the ducts is cold. [3]

Free checks (5 minutes, no tools)

Fan set to "On" instead of "Auto." Carrier calls this the most common cause. [1] On "On," the fan blows all the time, including when the burners are off. Trane: "The fan will blow cold air when the furnace isn't running." Switch it to Auto. [2]
Mode and temperature. Make sure the thermostat is on Heat, not Cool, and set a few degrees above the room temperature. [1][2] Carrier also says to check the schedule after a daylight saving time change or a power outage. [1]
Thermostat batteries. Trane and Lennox both list dying batteries. Lennox notes "Batteries don't have to be completely dead for the thermostat to malfunction." [2][3]
Dirty filter. A clogged filter can make the furnace overheat and shut the burners off as a safety step while the blower keeps running. [1][2] Turn the furnace off, put in a new filter, and turn it back on. Carrier says to replace 1-inch filters every 1–3 months, and Trane says every 30–90 days depending on the filter. [1][2]
Vents and returns. Open closed supply vents and clear furniture or rugs off return grilles. Carrier says blocked or closed vents can make the furnace overheat and trip the limit switch. [1]
Gas or fuel supply. For natural gas, Trane says to make sure the furnace gas valve is open: "the gas valve gets bumped." [2] Carrier suggests checking whether your other gas appliances work, and if they don't, call your utility. For propane or oil, check the tank level. [1]
Condensate drain (high-efficiency furnaces with white PVC pipes). A clogged condensate drain can keep the burners from lighting. [2][3] Lennox says to make sure the PVC pipe that leads to the floor drain is clear. [3] Trane recommends a technician clear the drain line, pan and condensate pump. [2] On the Rheem 90+ furnaces covered by its FAQ, a blocked drain switch "de-energizes the gas valve." [5]

Burners light, then shut off after a few seconds

If you can see the burners light and then go out while the blower keeps going, the usual suspects are:

Dirty flame sensor. Carrier's flame sensor page lists "Furnace starts, then shuts off after 2–5 seconds" as a sign. After repeated failed tries, the furnace can lock out. [4] Lennox recommends a dealer for this. [3]
Limit switch tripping from overheating, usually caused by poor airflow (filter, closed vents). Trane says repeated overheating can make the high-limit switch fail completely. Replacing it is a job for an HVAC technician. [1][2]
Burners or control board. Trane also lists clogged or cracked burners and control board problems, and says "Most issues will require the help of an HVAC professional." [7]

Lockout and reset. On the Rheem furnaces covered by its FAQ, one LED blink means a soft lockout ("No Flame Sense was detected after four ignition trials"), reset by turning off the 115-volt power or turning the thermostat off for 5–10 seconds. [5] Other brands differ, so check the code chart on your furnace door or in the manual. Carrier says to note the blinking LED pattern on the control board and call a pro, and to remove the cabinet door only if your owner's manual says you can. [1] If it locks out again after a reset, stop resetting and call.

Older furnace with a standing pilot

If your furnace has a pilot light and it's out, Carrier and Lennox say to relight it using the manufacturer's instructions on the furnace. If it won't stay lit, they point to a dirty pilot orifice or a bad thermocouple, which is a technician repair. [1][3] Lennox notes that most furnaces made since 2010 don't use pilot lights. [3]

When to stop and call an HVAC pro

Gas smell, soot around the furnace, or a CO alarm. Get out first. [1][6]
Burners keep shutting off after the free checks, or the furnace keeps locking out. [4][5]
Gas valve, burner, limit switch, flame sensor or control board work. [1][2][7]

Have the model and serial number from the furnace's rating plate and any blink code ready. Trane also warns that DIY repairs can void your warranty. [2]

Other no-heat guides

In an RV or camper, the furnace is a different system: see RV furnace and water heater not working. Truck cab heater: 12V truck fridge and heater troubleshooting. Water heater: gas water heater pilot won't stay lit.

We don't list part prices, repair costs, or flame-sensor readings. Those vary by model; check your furnace's manual.

Sources

General diagnostic guidance only, not specific to any single model. Sourced from the manufacturer and CPSC pages linked above. Last verified Oct 24, 2026.