# Furnace won't turn on: what to check first

Updated: 2026-10-24 · https://pros411.com/en/guides/hvac/furnace-wont-turn-on

## Short answer

Set the thermostat to "Heat", a few degrees above room temperature, and put in fresh batteries if the screen is blank. Make sure the furnace switch, which looks like a light switch on or near the furnace, is on. Reset a tripped furnace breaker once and replace a clogged filter. Then give it five minutes. Never open the burner compartment or try to light it yourself.

> Safety first: If you smell gas (like rotten eggs), do not touch any switch: get everyone out and call 911 or your gas company from outside. If a carbon monoxide alarm goes off, get out and call 911. Do not open the burner compartment or try to light the furnace.

## How to check a furnace that won't turn on

![Cartoon furnace in a closet, cold and silent, with its switch turned off and a glove reaching for it](https://pros411.com/img/guias/furnace-wont-turn-on.webp)

1. **No heat and no sound from the furnace.** The house is cold and the furnace stays quiet. These checks fix many no-heat calls and take about 15 minutes.
2. **Thermostat on Heat, a few degrees higher.** Set it to "Heat", three to five degrees above room temperature. Blank screen? Put in fresh batteries first.
3. **Turn on the furnace switch.** It looks like a light switch on the wall or the side of the furnace, and people flip it off by mistake. Set it to "On".
4. **Reset the furnace breaker once.** In the breaker panel, find the furnace breaker. If it sits in the middle or on "Off", push it fully off, then on. Only once.
5. **Replace a clogged filter.** A dirty filter can make the furnace overheat and shut itself off. If it looks gray, put in a new one of the same size.
6. **Give it five minutes.** You should hear it start and feel warm air within a few minutes. Still nothing? Note how the status light blinks and call a technician.

## High-efficiency furnace? Check the pipes outside

Furnaces with two white plastic pipes on an outside wall shut down when snow, ice or a nest blocks them. Clear snow away from the pipe ends by hand, gently, and keep the area clear. If the pipes come out on the roof, call a technician instead of climbing up.

## Why it happens

Most no-heat calls come down to power, a setting or airflow: dead thermostat batteries, a furnace switch turned off, a tripped breaker or a clogged filter. Inside the furnace, a failed ignitor, a dirty flame sensor or a stuck pressure switch also stop it. The status light behind the small window in the lower door blinks a code that tells a technician where to look.

## What not to do

- Do not open the burner compartment, remove panels or try to light the furnace.
- Do not reset the breaker again if it trips a second time. Something is shorting.
- Do not heat the house with the oven or a grill. They can fill it with carbon monoxide.
- Do not store paint, cleaners or boxes against the furnace.

## When to call an HVAC technician

- It still will not start after these checks, or the status light blinks a code.
- The breaker trips again after one reset.
- You hear clicking but it never lights, or it lights and shuts off within seconds.
- You smell something burning, or it bangs when it starts.
- Someone at home is at risk from the cold, or the pipes could freeze.

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## Frequently asked questions

### Why is my furnace not turning on?

Most often it is a thermostat setting or dead batteries, a furnace switch turned off, a tripped breaker or a clogged filter. You can check all of those yourself. A bad ignitor, flame sensor or control board needs a technician.

### Where is the furnace switch?

On the wall next to the furnace or on its side. It looks like a regular light switch, and in basements and closets it often gets turned off by mistake.

### How do I reset my furnace?

Turn it off at the furnace switch, wait a minute and turn it back on. That clears most lockouts. If it locks out again, call a technician instead of resetting it over and over.

### Can a dirty filter stop a furnace from turning on?

Yes. A clogged filter can make the furnace overheat, and a safety switch then shuts the burners off. Replace it, and check it every month in winter.

### Why is the light on my furnace blinking?

It is a code from the control board. The code chart is usually on a label on the furnace door. Write down the pattern and tell the technician.

### Is no heat an emergency?

It can be in freezing weather, especially for babies, older adults and people with health conditions, and the pipes can freeze. Use the "24/7 emergencies" filter to find technicians who answer now.