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If your furnace runs but blows cold air, check 5 things in order: thermostat set to “Heat” and “Auto” (not “On”), air filter (a clogged filter triggers the high-limit switch and shuts down burners), pilot light or ignitor, flame sensor (carbon buildup stops it from sensing the flame; cleaning or replacement runs $75–$250 per our cost guide), and gas supply. Only the thermostat setting and the filter swap are safe homeowner actions; the remaining three are technician-only because they involve the gas burner, ignition system, or gas line.
Your furnace is running but the air coming out of the vents is cold. In the middle of winter, this is not just uncomfortable — it can be dangerous, especially if temperatures drop below freezing overnight. A dirty air filter or a failed ignitor can each cause this: both prevent the burners from staying lit even though the blower fan keeps running. This guide covers one symptom; for every other furnace problem, see our complete furnace diagnostic guide.
Before you call for emergency service, run through these 5 checks. Two of them are safe homeowner actions that can resolve the problem in under 10 minutes. The other three tell you exactly what to report to the technician so they can bring the right parts on the first visit.
Furnaces burn natural gas or propane, producing carbon monoxide as a byproduct, and run blower motors on high-voltage electrical circuits. If you smell gas (a rotten egg or sulfur odor), do not touch any switches or electrical devices. Leave the house immediately, do not try to find the leak or shut off any gas valve, and once you are outside at a safe distance call 911 or your gas utility's emergency line. Do not re-enter until the utility or emergency responders say it is safe. Separately: do not try to kill the power by going to your service panel to flip the breaker for the furnace yourself — the technician will handle the full power-down and combustion-safety check on arrival. If you happen to have a heat pump instead of a gas furnace, refrigerant-side diagnosis is federally regulated under EPA Section 608, certification is legally required, and refrigerant work is never a homeowner task.
First: Check for a Gas Smell
Before troubleshooting anything, from wherever you happen to be in the house, notice whether you smell a rotten-egg or sulfur odor. Natural gas is odorless, but utilities add mercaptan (a sulfur compound) so a leak is detectable by smell from a normal distance — you do not need to approach the furnace itself to check.
- If you smell rotten eggs or sulfur: Do not flip switches, light matches, or use your phone inside, and do not touch any gas valve. Leave the house, then call 911 or your gas utility's emergency line from a safe distance outside (utility gas-safety guidance). This is a gas leak — not a furnace problem.
- If there is no gas smell: Proceed with the checks below.
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Check #1: Thermostat Settings
This sounds obvious, but it is easy to overlook — especially in homes with multiple people adjusting the thermostat or after a power outage resets the settings.
Verify all of these:
- Mode is set to HEAT — not COOL, AUTO, or FAN ONLY. In FAN ONLY mode, the blower circulates air without activating the burners.
- Set temperature is above current room temperature — the furnace will not fire if the thermostat thinks the house is already warm enough. Set it at least 3 to 5 degrees above the displayed room temperature.
- Batteries are not dead — a thermostat with a dim, flickering, or blank display cannot send a heat signal to the furnace. Replace the batteries as the thermostat's manual describes.
- No schedule override — programmable and smart thermostats may have lowered the set temperature based on a schedule. Check for "Away" or "Sleep" mode and override it manually.
Check #2: Air Filter
A clogged air filter is the first mechanical cause to rule out, because replacing it is a safe homeowner task. Here is why it matters:
When the filter is blocked, it restricts airflow across the heat exchanger. The heat exchanger is the metal chamber where combustion gases heat the air. Without enough air flowing over it, the heat exchanger overheats. To prevent cracking (which can leak carbon monoxide), the furnace has a high-limit safety switch that shuts off the burners when the temperature inside gets dangerously high.
The blower fan keeps running to cool the heat exchanger — but since the burners are off, it pushes cold air through your vents.
How to check:
- Locate the filter — usually in a slot between the return air duct and the furnace, or behind the return air grille on the wall.
- Pull it out and hold it up to a light. If you cannot see light through it, replace it immediately.
- After replacing, set the thermostat to OFF for 30 minutes to let the heat exchanger cool down.
- Set it back to HEAT. If it blows hot air, the filter was the problem. If it still blows cold air, leave the thermostat off and call a technician.
In cold climates like Chicago and Minneapolis, long heating seasons mean more hours of airflow through the filter. ENERGY STAR advises checking your filter every month, especially in winter.
Check #3: Pilot Light or Ignitor
Older furnaces may use a standing pilot light: a small flame that stays lit to ignite the burners. If it goes out, the burners have no ignition source and will not fire. Checking the pilot is part of the technician's diagnosis; you do not need to open or look inside the furnace.
Do NOT try to re-ignite the pilot yourself. While some old furnaces have re-ignition instructions printed on the access panel, the procedure involves working next to an open gas line and an open flame — it is a technician task, full stop. A qualified technician can safely restart the pilot and diagnose why it went out in the first place, including whether a safety part such as the thermocouple needs replacing.
If your furnace has an electronic ignitor, there is no pilot light. The ignitor is a small element that glows red-hot to ignite the gas. When it fails:
- You may hear the furnace click on, the inducer motor starts, but no ignition happens
- The furnace tries 2–3 times, then locks out
- The blower continues running, blowing cold air
A failed ignitor is a professional repair because it requires accessing the burner assembly. Our cost guide lists ignitor replacement at $100–$300.
Check #4: Flame Sensor
The flame sensor is a small metal rod positioned in the burner flame path. Its job is to confirm the flame is actually burning after the gas valve opens. If it cannot detect a flame within a few seconds, it shuts off the gas valve — even if the flame was burning fine.
The classic symptom: The furnace lights, runs for 30 seconds to 3 minutes, then the burners shut off and the blower pushes cold air. It may try to restart 2–3 times, then lock out completely.
Over time, the flame sensor develops a layer of carbon buildup that insulates it from the flame, making it unable to detect the heat. A technician can clean or replace the flame sensor as part of the service call. Our cost guide lists flame sensor cleaning or replacement at $75–$250.
Check #5: Gas Supply
If the furnace has power, the thermostat is calling for heat, the filter is clean, and the ignitor is trying to fire — but there is no flame at all — the gas supply may be interrupted.
What you can safely notice:
- Other gas appliances: Is your gas stove, water heater, or fireplace working? If none of them are, the issue is likely with your main gas supply — call your gas utility.
- Recent work: If a contractor, plumber, or the utility worked on your gas line or meter recently, tell the technician or the utility; the supply to the furnace may not have been restored.
Do NOT touch the gas valve, gas lines, or meter. Checking the furnace's gas shutoff valve is a technician task. If other gas appliances also stopped working, call your gas utility; otherwise, call an HVAC technician.
When to Call a Technician
Call an HVAC technician if:
- You replaced the filter and waited 30 minutes, but the furnace still blows cold air
- The furnace lights briefly then shuts off repeatedly (flame sensor or gas valve issue)
- You hear the ignitor clicking but no flame appears (failed ignitor or gas valve)
- The furnace will not turn on at all — no fan, no sounds, nothing
- You see an error code flashing on the furnace control board (note the flash pattern — the technician will need it)
- The furnace short cycles — turns on for a few minutes, shuts off, then restarts
Tell the technician: "The furnace blower runs but the burners are not staying lit" or "The furnace tries to ignite but shuts off after [X] seconds." This helps them diagnose faster and bring the right parts.
What the Repair Will Cost
These ranges come from our HVAC cost guide. Your actual cost will vary based on your region, the provider, and system specifics.
| Repair | Estimated Cost |
|---|---|
| Flame sensor cleaning or replacement | $75–$250 |
| Ignitor replacement | $100–$300 |
| Gas valve replacement | $200–$600 |
| Control board replacement | $300–$900 |
| Diagnostic / service call fee | $65–$150 |
Ranges from our cost guide (parts and labor). Actual costs vary by region, provider, and system.
If your furnace is down during a freeze, the American Red Cross advises opening kitchen and bathroom cabinet doors so warmer air reaches the plumbing and letting cold water drip from faucets served by exposed pipes. Call for service right away: burst pipes can add water damage on top of the furnace repair bill.
How to Prevent This Problem
- Check the filter every month and change it at least every 3 months. That is ENERGY STAR's advice, with extra attention in winter; in Denver and other cold-climate cities the heating season is long. Set a phone reminder.
- Schedule a fall tune-up. ENERGY STAR recommends a contractor check the heating system in the fall, including gas connections, gas pressure, burner combustion, and the heat exchanger. Ask the technician to include the ignitor and flame sensor. A seasonal tune-up runs $80–$200 per our cost guide.
- Install carbon monoxide alarms on each level of the home, outside each sleeping area, as the CPSC recommends (EPA summary). A faulty furnace can leak CO into the living space; CO is odorless, so you will not smell it, and it can be fatal.
- Know your furnace age. ENERGY STAR suggests talking to a contractor about replacement once a furnace is more than 15 years old; DOE's federal purchasing guidance assumes an average furnace life of 21 years, a planning figure rather than a promise. If yours is past 15 and needing frequent repairs, start planning for replacement before it fails during the coldest week of the year.
- Keep supply and return vents clear. Do not block vents with furniture, rugs, or curtains. Restricted airflow causes the same overheating problems as a dirty filter.
Frequently Asked Questions
Possible causes include incorrect thermostat settings, a dirty air filter restricting airflow, an extinguished pilot light or failed ignitor, a dirty flame sensor, or a malfunctioning gas valve. Start by checking your thermostat is set to HEAT (not COOL or FAN ONLY) and that the temperature is set above current room temperature. Then check the air filter — a severely clogged filter can cause the furnace to overheat and shut down the burners as a safety measure.
Yes. A clogged air filter restricts airflow across the heat exchanger. When the heat exchanger cannot dissipate heat fast enough, it overheats and triggers the high-limit safety switch, which shuts down the burners. The blower fan continues running to cool the heat exchanger, but since the burners are off, it pushes unheated air through the vents. Replacing the filter and setting the thermostat to OFF for 30 minutes so the furnace can cool down may resolve this.
It depends on the cause. A new air filter is an inexpensive part you can replace yourself. Per our cost guide, flame sensor cleaning or replacement runs $75 to $250, an ignitor replacement runs $100 to $300, and a gas valve replacement runs $200 to $600. A diagnostic or service-call fee typically runs $65 to $150; some providers credit it toward the repair if you book it with them, so ask when you call.
If you smell gas, leave the house right away. Do not touch switches, the thermostat, or any gas valve, and do not try to find the leak. Once you are outside at a safe distance, call 911 or your gas utility's emergency line, and do not go back in until they say it is safe. If there is no gas smell, you can leave the furnace on while you check the thermostat and filter. If the furnace keeps turning on and off in short bursts (short cycling), set the thermostat to OFF and call a technician.
This is a classic sign of a dirty flame sensor or an overheating heat exchanger. The flame sensor is a small rod that detects whether the burner flame is present. When it gets coated with carbon buildup, it cannot detect the flame reliably and shuts off the gas valve as a safety precaution — even though the flame was burning normally. The blower continues running, pushing cold air. A dirty air filter can also cause this by triggering the high-limit switch after a few minutes of operation.
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