You hear the furnace try — the inducer fan spins up, maybe a click or two — but the burners never catch, or they light for a few seconds and drop right back out. Then the house stays cold. A furnace that won't ignite is one of the most common no-heat calls of the season across the Peninsula and older East Bay, and it usually comes down to one ignition part or a gas-supply problem, not a dead furnace.
This page covers why a furnace won't light, what you can safely check yourself, and when it's time for a licensed tech.
How it usually shows up:
- You hear a click or the inducer fan spin up, but the burners never light.
- The furnace lights, runs a few seconds, then shuts off and tries again.
- There's no glow behind the panel where the igniter should heat up.
- After a few attempts it stops completely and won't retry until it's reset.
Why your furnace won't ignite
When a furnace tries but won't light, it's almost always one of these — roughly in the order we work through them:
- A failed hot-surface igniter. Very common. This brittle element glows red-hot to light the gas; when it cracks or burns open it stops glowing, and the gas never lights.
- A dirty or misaligned flame sensor. The furnace lights, but a sensor coated in residue can't confirm the flame, so the control board shuts the gas off within seconds — again and again.
- Dirty burners. Rust, dust, or debris across the burners causes weak, delayed, or rolling ignition instead of a clean light.
- Gas supply off or a failing gas valve. A closed shutoff, an empty tank, or a valve that won't open leaves the igniter glowing with no gas to light.
- A blocked flue or a pressure-switch fault. The pressure switch has to close to allow ignition; if the flue or inducer path is blocked, it won't, and the sequence stops before the burners fire.
- The ignition control or board. The board that sequences ignition can itself fail and never energize the igniter or valve.
- An ignition lockout. After a set number of failed attempts, the board locks out on purpose and won't try again until power is cycled.
The usual fix
Our tech follows the furnace's sequence of operations to find exactly where ignition breaks: the inducer starts, the pressure switch closes, the igniter glows, the gas valve opens, and the flame sensor proves the flame. Whichever step fails points straight to the part. Most won't-ignite calls end with a new hot-surface igniter, a cleaned or replaced flame sensor, or a gas-valve or control-board repair. You get a firm, flat-rate quote before any part goes in.
DIY vs. calling a pro — the honest version
Safe to check yourself first (free):
- Confirm the gas is on. Make sure the gas shutoff is open and that other gas appliances — your stove or water heater — still work, which tells you gas is reaching the house.
- Change a dirty air filter. Poor airflow can nudge the furnace into overheating and lockouts, so start clean.
- Cycle the power once. Switch the furnace off at its power switch or breaker, wait a moment, and switch it back on to clear a one-time lockout.
Call a pro when:
- The igniter, flame sensor, or gas valve needs testing or replacing.
- The furnace keeps locking out after you've reset it.
- You smell gas at all — leave the house and call your gas utility or 911 first.
Repeatedly forcing a furnace to try to light on a fault is hard on the igniter and the gas valve. After one or two resets with no lasting flame, stop and call — the ignition train and line voltage are where a licensed tech keeps you and your warranty safe.
What it costs in the Bay Area
Ignition repairs are usually mid-range parts, not a new furnace. In our pricing, a hot-surface igniter runs about $120–$350 and a flame sensor $90–$250 — the common won't-ignite fixes — while a control board is roughly $200–$600 if the board itself has failed. Cleaning a flame sensor or clearing a lockout sits at the low end. We charge a diagnostic fee that's credited toward the repair when you approve the work, always with a firm, upfront price first.
Get your furnace lighting again
Triton repairs gas and electric furnaces across the Peninsula and East Bay as part of our full HVAC repair service — San Mateo, Burlingame, Redwood City, Alameda, San Leandro, and Hayward — where coastal damp and older homes are hard on igniters and flame sensors that only run a few months a year. Same-week and often same-day appointments, upfront pricing, licensed techs.
Book online or call (888) 993-5699. We'll find why it won't light and get the burners firing again.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- Furnace Not Heating / No Heat — no heat, from the top
- Furnace Blowing Cold Air (Not Hot) — runs but the air is cold
- Carrier Furnace Code 34 (Ignition Proving Fault) — the ignition-proving code
- Pilot Light Won't Stay Lit — older standing-pilot furnaces
- HVAC & Furnace Repair in Redwood City — Peninsula service area
Frequently asked questions
Why won't my furnace ignite? The most common reason is a failed hot-surface igniter that no longer glows to light the gas. Close behind are a dirty flame sensor that makes the board shut the gas off seconds after lighting, dirty burners, no gas reaching the valve, or a blocked flue that keeps the pressure switch from closing. Confirm the gas is on, then have the ignition parts tested.
Why does my furnace click but not light? The click is often the gas valve or ignition control trying to start the sequence while the burners never catch — usually because the hot-surface igniter isn't glowing or no gas is arriving. On older systems a repeated click can be a spark igniter that isn't lighting the pilot. Either way, if there's no flame after a couple of tries, it needs a tech.
Why does my furnace light and then shut off right away? That's the classic sign of a dirty or misaligned flame sensor. The burners light, but the sensor can't prove the flame to the control board, so the board shuts the gas off within seconds as a safety — then tries again and repeats. Cleaning or realigning the flame sensor clears many of these; a tech does it safely.
What is an ignition lockout and how do I clear it? After a set number of failed light attempts, the control board locks the furnace out on purpose so it doesn't keep dumping gas. You can usually clear a one-time lockout by cycling the power — off at the switch or breaker, wait a moment, back on. If it locks out again, something real is wrong and it needs to be diagnosed, not repeatedly reset.
Is it safe to keep trying to light my furnace? Resetting once is fine. But repeatedly forcing a furnace that won't stay lit keeps sending gas to a burner that isn't proving a flame, and it's hard on the igniter and gas valve. After one or two resets with no lasting heat, stop and call. If you smell gas at any point, leave the house and call your gas utility or 911 first.
How much does it cost to fix a furnace that won't ignite? It depends on the part: a hot-surface igniter is about $120–$350 and a flame sensor $90–$250, while a control board runs roughly $200–$600 if the board has failed. Cleaning a flame sensor or clearing a lockout sits at the low end. We credit the diagnostic fee toward the repair and give you a firm price before any work begins.