Short answer: Check the thermostat first: Trane notes that when the fan is set to ON instead of AUTO, the blower keeps running between heating cycles and blows unheated air. Next, replace the filter, because a clogged filter can make the furnace overheat and shut off the burners. If those are fine, the flame sensor, igniter, gas supply or (on high-efficiency furnaces) a blocked condensate line may be keeping the burners from running.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Cold air between warm cycles | Thermostat fan set to ON | Set fan to AUTO | No |
| Warm, then cold after a few minutes | Overheating from a dirty filter (limit switch) | Replace the filter | If it continues |
| Burners light, then shut off; blower keeps going | Dirty flame sensor | Call for cleaning | Yes |
| No flame at all | Igniter, pilot or gas supply | Check gas valve open, propane/oil level | Yes |
| High-efficiency furnace stops heating | Clogged condensate line | Look for water around the furnace | Yes |
| Some rooms cold, others fine | Duct leaks or closed vents | Check vents are open | If it persists |
Thermostat and fan
Set the thermostat to Heat, the fan to AUTO and the temperature above room temperature. Replace the batteries if needed.
Filter and overheating
A clogged filter restricts airflow. The furnace overheats, the high-limit switch shuts the burners off, and the blower keeps pushing air that is no longer heated. Trane suggests changing filters every 30 to 90 days. Turn the furnace off before you change it.
Fuel supply and ignition
- Natural gas: make sure the gas valve to the furnace is open.
- Propane or oil: check the tank level.
- Older furnaces: a pilot that went out can be relit following the label on the unit.
- Newer furnaces: a failed igniter or a dirty flame sensor needs a technician.
When to call a professional
- The burners do not light, or light and shut off within seconds.
- The furnace keeps overheating after a new filter.
- There is water around a high-efficiency furnace.
- You smell gas: leave the house and call the gas company.
What it usually costs
HomeGuide (2026) lists flame sensor replacement at about $80 to $250 and igniter replacement at about $100 to $250. Most furnace repairs fall between $125 and $480.
Frequently asked questions
Why is my furnace blowing cold air but the pilot is on?
The burners may not be lighting from the pilot, the flame sensor may be shutting them off, or the limit switch may be cutting heat due to overheating.
Is it normal for a furnace to blow cold air at first?
A short delay is normal: many furnaces wait until the heat exchanger warms before the blower starts. Steady cold air is not.
Related problems
Sources
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.