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HVAC guide

Furnace not turning on or not heating: what to check first

By the Home Doctor team · Reviewed

Quick answer

When a furnace won't turn on or blows cold air, check the easy things first: the thermostat set to Heat with fresh batteries, the power switch and breaker on, a clean filter, and the front panel on tight. If those check out, the usual culprits are a dirty flame sensor, a failed igniter, or a clogged condensate drain on a high-efficiency furnace, typically $75 to $450 to fix. If a carbon monoxide alarm sounds or you smell gas, get everyone out and call 911 or your gas utility from outside.

How urgent
Urgent

Urgent in cold weather, especially with babies, older adults, or pipes at risk of freezing. 'Stop and call a pro now' if a CO alarm sounds or you smell gas.

DIY or pro
DIY if handy

Thermostat, filter, switch, breaker and condensate checks are DIY. Cleaning a flame sensor is a handy job. Gas valves, burners, control boards and heat exchangers are for a licensed HVAC tech.

Stop and call a pro now if

  • A carbon monoxide alarm is sounding, or people feel headachy, dizzy or sick when the heat runs: get everyone out, call 911 from outside, and don't go back in until responders say it's safe.
  • You smell gas or rotten eggs: don't touch switches or the thermostat. Leave the house and call your gas utility or 911 from outside.
  • Soot or scorch marks on the furnace, or flames outside the burner area: turn it off and call a technician.
  • A rollout switch (a small safety switch near the burners, often with a reset button) has tripped: don't keep resetting it. It can mean a blocked flue or a cracked heat exchanger, which can leak carbon monoxide.

Common causes and how to tell them apart

  1. 1. Thermostat

    How to tell
    The display is blank, it's set to Cool or Off, or the setpoint is below room temperature.
    What fixes it
    Set it to Heat, a few degrees above room temperature. Replace the batteries.

  2. 2. Power switch or breaker off

    How to tell
    Nothing happens at all: no fan, no click, no status light. The furnace switch, which looks like a light switch on or near the unit, is off, or the breaker is tripped.
    What fixes it
    Turn the switch on and reset the breaker once. If the breaker trips again, call a technician.

  3. 3. Clogged filter tripping the high-limit switch

    How to tell
    Weak airflow. The burners light, run a few minutes, then shut off while the blower keeps going.
    What fixes it
    Replace the filter and open any closed supply vents. If it keeps shutting off, a tech needs to find out why it's overheating.

  4. 4. Pilot out or igniter failed

    How to tell
    On most furnaces, a small draft fan starts but the burners never light, and the igniter doesn't glow. On an older furnace, the pilot is out.
    What fixes it
    Relight a standing pilot by following the label on the furnace. A cracked or dead igniter needs replacing.

  5. 5. Dirty flame sensor

    How to tell
    The burners light, then shut off within a few seconds. It tries a few times, then locks out.
    What fixes it
    With the power off, remove the sensor rod and clean it gently with a fine abrasive pad. A tech does it in minutes.

  6. 6. Clogged condensate drain (high-efficiency furnaces)

    How to tell
    A furnace with white plastic vent pipes won't fire or keeps shutting off, sometimes with water around the base.
    What fixes it
    Clear the condensate line and trap. A safety switch keeps the furnace off while the water can't drain.

  7. 7. Control board or blower motor

    How to tell
    The status light flashes an error code, or the burners light but no air comes out of the vents.
    What fixes it
    Look up the code on the chart inside the panel door and give it to the tech. Boards and motors are a technician's job.

Not sure which one you have?

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What you can safely try first

  1. 1Set the thermostat to Heat, a few degrees above room temperature, and replace its batteries.
  2. 2Check the furnace power switch and the breaker, and replace the air filter.
  3. 3Make sure the furnace's front panel is on tight. A safety switch keeps it off when the door is loose.
  4. 4On a high-efficiency furnace, check the white plastic pipes outside for snow, ice or debris, and look for water around the base.
  5. 5Note any blinking status light pattern, then switch the furnace off for about 30 seconds and back on to clear a lockout. Once, not over and over.

When to call a pro

  • The burners won't light, or they still shut off after you've cleaned the flame sensor.
  • It locks out again or flashes the same error code after a reset.
  • A limit or rollout switch keeps tripping.
  • Any gas smell, CO alarm, soot or scorching.

What it costs

Typical costs to fix: furnace not heating
JobDIY partsPro, installed
Replace air filter$10–$40—
HVAC diagnostic visit—$75–$200
Clean or replace flame sensor$0–$40$75–$300
Clear condensate drain or trap$0–$20$100–$250
Replace hot surface igniter$20–$80$150–$450
Replace control boardNot recommended$300–$900

Ballpark US ranges as of September 27, 2026. Your region, access and the specifics of your home can move these a lot. Get two or three quotes for anything over a few hundred dollars.

What to tell the pro

My [gas / propane] furnace, about [age] years old, [won't turn on at all / runs but blows cold air / lights and shuts off after a few seconds]. It's a [standard / high-efficiency with white plastic vent pipes] model. I've checked the thermostat, switch, breaker and filter. The status light is flashing [pattern or code]. There's no gas smell and no CO alarm. Can you give me an itemized price before you start?

Questions people ask

Why is my furnace running but blowing cold air?

Either the burners aren't lighting or staying lit, or the thermostat fan is set to On instead of Auto, which runs the blower between heating cycles. Set the fan to Auto first. If the air stays cold, the usual causes are a dirty flame sensor, a failed igniter, a clogged condensate drain, or the high-limit switch tripping from a clogged filter.

How do I reset my furnace?

Switch off the furnace power switch or breaker for about 30 seconds, then turn it back on. That clears a lockout on most modern furnaces. If it locks out again, don't keep cycling it. Call a technician and give them the error code.

How do I keep the house warm until the furnace is fixed?

Close the doors to rooms you aren't using. In freezing weather, open the cabinet doors under sinks and let faucets drip to protect pipes. Keep space heaters 3 feet from anything that can burn and plug them straight into the wall. Never heat the house with a gas oven or stove, a grill or a generator: they can produce deadly carbon monoxide.

What should I do if my carbon monoxide alarm goes off?

Get everyone, including pets, outside into fresh air right away and call 911 from outside. Don't go back in until the fire department says it's safe. Carbon monoxide has no color or smell, so the alarm may be your only warning. Have a licensed HVAC tech inspect the furnace and other fuel-burning appliances before you use them again.

Get an answer for your house

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General guidance, not a substitute for a licensed professional who can see the problem in person. If you smell gas, see sparks or smoke, or anyone is hurt, leave the area and call 911.