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Fix a furnace that blows cold air

We'll confirm the blower runs, rule out thermostat and filter issues, then isolate the cause—igniter, flame sensor, or gas valve for gas; sequencer or heating elements for electric—or tell you when to call a pro.

Troubleshooting · Home appliances
15–45 min
What you'll need
  • Multimeter (for continuity tests)
  • Screwdriver set (Phillips and flathead)
  • Steel wool or fine emery cloth (for cleaning flame sensor)
  • Replacement filter, igniter, or flame sensor (if needed)
Step 1 of 9

Quick triage — pick your path

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Choose the option that matches what you see. You can jump straight to that section.

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Steps

Goal: Confirm the symptom, rule out thermostat and filter, then isolate the heating fault.

  • Set the thermostat to Heat and raise the temperature above the room reading. Wait for the blower to start and feel the supply vents after two to three minutes.
  • Good: The blower runs but the air stays cold—heating system has failed. Proceed to Check thermostat and filter.
  • Bad: The blower does not run—different problem (thermostat, blower motor). See When to get help.

Check thermostat and filter

Goal: Rule out thermostat settings and filter blockage before opening the furnace.

  • Confirm the thermostat is set to Heat (not Cool or Off) and the temperature is above the current room temperature. Replace batteries if the thermostat uses them.
  • Locate the filter (usually in the return duct or at the furnace). Replace it if dirty—use the size printed on the old filter.
  • Check the circuit breaker for the furnace and the furnace switch (often near the unit). Both must be on.
  • Good: Thermostat on Heat, filter clean, power on. Proceed to Gas path or Electric path based on your furnace type.
  • Bad: Thermostat wrong, filter severely dirty, or breaker keeps tripping—fix those first, or call a pro.

Gas path

Goal: Inspect the igniter, clean the flame sensor, or call a pro for gas valve work.

  • Turn off power at the breaker and shut off the gas supply at the valve. Open the burner compartment per your model. Check your owner’s manual or search “[brand] [model] furnace service manual” for access steps.
  • Locate the igniter. Inspect for cracks or breaks. Test for continuity if possible. If cracked or failed, replace it. Restore power and gas—the igniter should glow and the burner should light.
  • If the furnace lights briefly then shuts down, clean the flame sensor. Locate the thin metal rod near the burner with a single wire. Remove it and gently clean with steel wool or fine emery cloth. Reinstall and test.
  • Good: The igniter glows and the flame lights, or the furnace lights and stays lit after cleaning the flame sensor.
  • Bad: The igniter glows but no flame appears—gas valve issue. Call an HVAC technician. Do not work on gas valves yourself.

Electric path

Goal: Test and replace the sequencer or heating elements on an electric furnace.

  • Turn off power at the breaker. Open the furnace access panel. Locate the sequencer and heating elements.
  • Test the sequencer for continuity when cold. Test each heating element for continuity. Replace any that show no continuity.
  • Good: You found and replaced the faulty part. Restore power and test—the furnace should heat.
  • Bad: All parts test good but the furnace still does not heat—call an HVAC technician. If you are not comfortable working with 240 volts, call a pro.

When to get help

If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside. Do not call a technician for an active gas leak—that is an emergency.

Call an HVAC technician if:

  • The furnace is gas and the igniter glows but no flame appears (gas valve).
  • The furnace lights then shuts down and cleaning the flame sensor did not help.
  • You have replaced the sequencer and heating elements and it still does not heat.
  • The blower does not run (thermostat or blower motor—different problem).
  • You are not comfortable working with electricity or gas.

Verification

  • The blower runs and the supply air is warm within three to five minutes of the thermostat calling for heat.
  • No error codes or unusual noises.
  • The igniter, flame sensor, sequencer, or heating elements (if replaced) work correctly and the furnace heats consistently.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify the blower runs but the air stays cold; rule out a different problem.
  2. Thermostat and filter Check thermostat mode and temperature; replace dirty filter.
  3. Gas — igniter and flame sensor Inspect and replace igniter; clean or replace flame sensor.
  4. Electric — sequencer and elements Test and replace sequencer or heating elements if faulty.
  5. Call a pro Gas smell—evacuate, call 911 or gas utility from outside. Gas valve work or repeated failures—call an HVAC technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Furnace type (gas or electric)
  • Whether the blower runs
  • Thermostat mode and setting
  • Filter condition
  • Igniter / flame sensor / sequencer test results
  • Steps already tried

Does the blower run but the air stay cold?

Set the thermostat to Heat and raise the temperature. Wait for the blower to start and feel the supply vents after two to three minutes. If the blower runs but no warmth, the heating system has failed.

Set the thermostat to Heat and raise the temperature above the room reading. Wait for the blower to start. Feel the supply vents after two to three minutes. Good: blower runs but air stays cold—heating fault. Bad: blower does not run—different problem (thermostat, blower motor); call a technician.

Yes No

You can change your answer later.

Is the thermostat on Heat and the filter clean?

Wrong thermostat mode or a dirty filter can prevent heat. Check both before opening the furnace.

Confirm the thermostat is set to Heat (not Cool or Off) and the temperature is above room temp. Replace batteries if applicable. Check the filter—replace if dirty. Good: thermostat on Heat, filter clean. Bad: thermostat wrong or filter severely dirty—fix those first.

Yes No

You can change your answer later.

Is the furnace gas or electric?

Gas furnaces have a gas line and vent pipe. Electric furnaces have a power connection and heating elements.

Check the furnace. Gas: gas line, vent pipe, burner assembly. Electric: thick power cord or junction box, heating elements. Good: you know the type. Bad: unsure—call a technician.

Gas Electric

You can change your answer later.

Is the igniter cracked or showing no continuity?

The igniter glows when the furnace calls for heat. A cracked or failed igniter prevents the burner from lighting.

Turn off power and gas. Open the burner compartment. Locate the igniter. Inspect for cracks. Test for continuity if possible. Cracked or no continuity: replace igniter, restore power and gas, test. Good: igniter glows and flame lights. Bad: igniter glows but no flame—gas valve issue; call a technician. Do not work on gas valves yourself.

Yes No

You can change your answer later.

Replace igniter and test

Replace the igniter with a matching part. Restore power and gas. The igniter should glow and the burner should light. If the igniter glows but no flame appears, the gas valve has failed—call a technician.

Done

Does the furnace light then shut down?

A dirty flame sensor can cause the furnace to light briefly then shut down. Cleaning often fixes it.

If the furnace lights then shuts down, clean the flame sensor. Turn off power and gas. Locate the flame sensor (thin rod near the burner). Remove and clean with steel wool or emery cloth. Reinstall and test. Good: furnace lights and stays lit. Bad: still shuts down—replace flame sensor or call a pro. If igniter glows but no flame—gas valve; call a technician.

Yes No (igniter glows, no flame)

You can change your answer later.

Clean flame sensor and test

Clean the flame sensor with steel wool or fine emery cloth. Reinstall and test. The furnace should light and stay lit. If it still shuts down, replace the flame sensor or call a technician.

Done

Does the sequencer or heating element have continuity?

The sequencer turns on the heating elements. Test both for continuity.

Turn off power. Open the furnace. Locate the sequencer and heating elements. Test for continuity. Replace any that fail. If you are not comfortable with 240 volts, call a technician. Good: replaced faulty part—furnace should heat. Bad: all test good but still no heat—call a technician.
Question

Does the sequencer or heating element have continuity?

Done

Call a technician

If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside. Call an HVAC technician if: the blower does not run; the gas igniter glows but no flame (gas valve); the furnace lights then shuts down and cleaning the flame sensor did not help; you have replaced the sequencer and heating elements and it still does not heat; or you are not comfortable working with electricity or gas. Never work on gas valves or gas lines yourself.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a furnace blow cold air?
Gas furnaces: a failed igniter, dirty or faulty flame sensor, or gas valve. Electric furnaces: a bad sequencer or heating element. A dirty filter or wrong thermostat setting can also cause cold air. Check thermostat and filter first, then the heating components.
Can I fix a furnace that blows cold air myself?
Yes, you can check the thermostat, replace the filter, and clean the flame sensor on gas furnaces. You can inspect and replace the igniter. Gas valve work always requires a professional. Never work on gas lines yourself.
When should I call a technician for a furnace that blows cold air?
If you smell gas, evacuate immediately and call 911 or your gas utility from outside. Call an HVAC technician if the igniter glows but no flame appears (gas valve), if the furnace lights then shuts down and cleaning the flame sensor did not help, if you have replaced the sequencer and heating elements and it still does not heat, or if you are not comfortable working with electricity or gas.

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