Generic HVAC Furnace Blowing Cold Air: How to Fix It Yourself
When your furnace blows air but it is not warm, the burners are either not lighting or they light briefly then shut off. The most common fix is cleaning the flame sensor — a small metal rod that tells the control board the burners are lit. When it gets dirty, the furnace lights for a few seconds and then shuts down, leaving only the blower running.
⚠️ Safety First
- If you smell gas at any point, stop what you are doing, leave the house, and call the gas company.
- Turn off the furnace power switch before opening panels or working on any components.
- Let the furnace cool down before touching internal parts — the heat exchanger and other parts can burn you.
Parts You May Need
Step-by-Step Repair
- 1
Check the thermostat settings
Make sure the thermostat is set to Heat and the fan is set to Auto (not On). When the fan is set to On, it runs continuously even when the burners are off. This blows unheated air and feels cold. Switching to Auto so the fan only runs during heating cycles may fix the problem immediately.
- 2
Check the air filter
A very dirty filter can cause the furnace to overheat. When the furnace overheats, the high-limit switch shuts off the burners but the blower keeps running to cool things down. Pull out the filter and replace it if it is dirty. This is a common overlooked cause.
- 3
Watch the burner ignition sequence
Turn the furnace back on and watch through the viewing window. You should see the ignitor glow, then the burners light. If the burners light for a few seconds and then go out while the blower keeps running, the flame sensor is almost certainly dirty.
- 4
Clean the flame sensor
Turn off the furnace power switch. Remove the front panel. The flame sensor is a thin metal rod (usually with a single wire attached) positioned near the burner flames. Remove the single screw holding it, pull it out, and gently sand the metal rod with fine-grit sandpaper or emery cloth until it is shiny. Do not use regular sandpaper that leaves grit behind. Put it back and tighten the screw.
- 5
Reassemble and test
Put the panel back on, turn on the power switch, and set the thermostat to call for heat. Watch the ignition cycle again. The burners should light and stay lit this time. Give it a few minutes and you should feel warm air coming from the vents. If the burners still cut out, the flame sensor itself may need to be replaced.
If That Didn't Work
- Dirty or cracked ignitor that is not lighting the gas
- Gas valve not fully open
- Cracked heat exchanger (dangerous — produces cold spots and can leak carbon monoxide)
- Control board malfunction
When to Call a Professional
If cleaning the flame sensor does not solve it, or if you see any cracks in the heat exchanger, call a technician right away. A cracked heat exchanger can leak carbon monoxide into your home — this is a serious safety issue. Also call a pro if the gas valve seems to be the problem, as gas work should only be done by licensed professionals. Expect $100–$250 for a service call.
Related Repairs
Disclaimer: This guide is for informational and educational purposes only and does not constitute professional repair advice. All repairs are performed at your own risk. RepairItFree.com is not liable for any personal injury, property damage, device damage, voided warranties, or other losses resulting from the use of this information.
If you are unsure about any step — especially anything involving electricity, gas, water, or refrigerant — stop and consult a qualified professional. Your safety is more important than saving money.
See our full Terms of Service for complete legal details.