Furnace Won’t Ignite? Pilot, Igniter and Gas Problems Explained

Quick answer: When a gas furnace will not ignite, the usual causes are a cracked or worn hot surface igniter, no gas reaching the burner, a pressure switch or draft inducer problem, a dirty flame sensor that makes it shut off right after lighting, or a control board lockout after several failed tries. Check the thermostat, power switch, breaker, filter, door panel and whether other gas appliances work, then read the blinking light code on the furnace. If it still will not light, call for furnace repair. If you smell gas, leave the house and call 911 or your gas utility from outside.

A furnace that will not light is one of the most common no-heat calls we get, especially on the first cold mornings of the season. The good news is that the cause is often a single part. This guide walks through how a modern furnace lights, what can go wrong at each step and what you can safely check yourself.

How a modern gas furnace lights

Understanding the sequence makes the problem much easier to pin down. When the thermostat calls for heat, a typical furnace does this:

1. Draft inducer starts. A small fan pulls air through the heat exchanger and pushes exhaust out the vent.
2. Pressure switch closes. It proves the inducer is moving air and the vent is clear.
3. Igniter heats up. A hot surface igniter glows orange (some furnaces use a spark instead).
4. Gas valve opens. Gas flows to the burners and lights from the igniter.
5. Flame sensor proves the flame. If it cannot detect a flame within a few seconds, the gas valve closes.
6. Blower starts. After a short delay, warm air flows through the ducts.

If any step fails, the furnace stops, usually tries again a few times, and then locks out to protect your home.

Cause 1: A cracked or worn igniter

Hot surface igniters are made of a brittle material that weakens with every heating cycle. Over the years they crack or lose their ability to get hot enough. Signs:

• You hear the inducer start but never see an orange glow through the sight window
• The igniter glows dimly or only partly
• The furnace runs fine some days and fails on others

Replacing an igniter is a common and usually minor repair. Igniters should never be touched with bare fingers, because skin oils create hot spots that shorten their life, so this is a job for a technician.

Cause 2: No gas reaching the burner

• Gas shutoff valve near the furnace turned off by accident
• Gas meter or service issue (check whether your stove or water heater works)
• Failed gas valve inside the furnace
• Propane tank empty in homes heated with propane

If none of your gas appliances work, contact your gas utility or propane supplier. If you smell gas at any point, leave and call from outside.

Cause 3: Pressure switch or draft inducer problems

If the inducer does not run, or the pressure switch never closes, the furnace will not even try to light. Common causes:

• A blocked vent or intake pipe (snow, ice, leaves or a bird nest on high-efficiency furnaces that vent through the wall)
• A cracked or water-filled pressure switch hose
• A clogged condensate drain that backs water into the inducer housing (see furnace leaking water)
• A failed inducer motor (often loud or humming before it fails)

After snowstorms, check that the white plastic vent pipes outside are clear. See snow and ice around vents.

Cause 4: The furnace lights, then shuts off

If you see a flame for a few seconds and then it goes out, the most likely cause is a dirty flame sensor. The furnace cannot prove the flame and shuts the gas off for safety. Cleaning or replacing the sensor fixes it. Read dirty flame sensor.

Cause 5: Lockout after repeated failures

After several failed ignition attempts, most furnaces enter a lockout for an hour or more. Resetting power will often let it try again, but if it locks out repeatedly, there is a real fault to find. Repeatedly resetting a furnace that is failing to light is not a good idea.

Older furnaces with a standing pilot

Furnaces from decades ago may have a pilot light that burns all the time. Causes of a pilot that will not stay lit include a dirty pilot orifice, a failed thermocouple, drafts or a gas supply problem. If your furnace has a standing pilot, it is likely old enough that a replacement conversation is worthwhile. Follow the lighting instructions on the furnace label exactly, and stop if it will not stay lit.

Safe checks you can do

1. Thermostat: set to Heat, a few degrees above room temperature, with fresh batteries (see thermostat not working)
2. Power: the furnace service switch (looks like a light switch) on, and the breaker not tripped
3. Door panel: fully closed; a safety switch stops the furnace if it is loose
4. Filter: not clogged
5. Gas: other gas appliances working; gas valve handle in line with the pipe
6. Vents outside: clear of snow, ice and debris
7. Error light: count the blinks on the control board and compare with the chart on the furnace door
8. Reset once: power off for 30 seconds, then on

If the furnace still will not light, it is time to call. See the full furnace not working checklist.

What not to do

• Do not repeatedly reset a furnace that keeps locking out
• Do not bypass safety switches or jumper wires
• Do not try to light burners with a lighter or match on a modern furnace
• Do not remove the burner cover or gas valve
• Do not ignore a gas smell, sooty marks or a yellow, lazy flame

How a technician finds the cause

We follow the ignition sequence step by step: confirm the call for heat, watch the inducer, test the pressure switch, measure the igniter, check gas pressure, test the flame sensor signal and read the control board's fault history. Then we explain the problem and give you a written price before any work. We also check venting and carbon monoxide while we are there.

How to prevent ignition problems

• Yearly heating tune-up before the cold arrives: cleaning the flame sensor and burners, testing the igniter and checking the inducer and pressure switch
• Change filters regularly
• Keep vents clear of snow and leaves
• Keep the condensate drain clear on high-efficiency furnaces

Our $89 heating tune-up is available this season, and the Prime Comfort Club covers yearly visits for $19/month (terms apply).

Repair or replace?

An igniter or flame sensor is a minor fix, even on an older furnace. If your furnace is 15 to 20 years old and needs a gas valve, inducer or control board on top of other recent repairs, compare repair with replacement. See furnace repair cost factors and signs you need a new furnace.

When the cold makes it urgent

If it is very cold, and especially if older relatives, infants or anyone with health conditions live in the home, treat a furnace that will not light as an emergency. Protect your pipes while you wait (see no heat in freezing weather) and call our 24/7 line.

Same day furnace repair

We repair every brand of gas furnace with same day service, upfront pricing and 24/7 emergency help across Ocean, Monmouth, Burlington and Middlesex counties, including Brick, Howell, Manchester and Woodbridge. New customers get $50 off their first repair (terms apply).