7 Reasons Your Gas Furnace Won't Ignite
Faulty or Worn Ignitor
Very CommonHot surface ignitors (HSI) are the most common ignition system in modern gas furnaces. They glow red-hot to ignite the gas. Over time, they crack, weaken, or fail completely.
Symptoms:
- No glow visible when furnace tries to start
- Ignitor glows but is dim orange instead of bright
- Visible cracks or damage on the ignitor element
- Furnace clicks/hums but burners never light
DIY Fix?
No - Call a Pro
Typical Cost
Call for quote
Likelihood
Very Common
Pro Tip: Ignitors typically last 3-5 years. If yours is older and failing intermittently, replacement is the best option.
Dirty Flame Sensor
Very CommonThe flame sensor is a small metal rod that detects whether the burner is lit. When dirty, it can't sense the flame and shuts off gas as a safety measure—even if the flame is actually burning.
Symptoms:
- Furnace ignites briefly then shuts off within 3-10 seconds
- You see flame, then it goes out immediately
- Furnace attempts to restart multiple times
- Error code indicating flame failure
DIY Fix?
Possible with caution
Typical Cost
Call for quote
Likelihood
Very Common
Pro Tip: This is the #1 service call for "furnace lights then goes out." A simple cleaning often fixes it, but it involves working near gas components.
Gas Valve Problems
CommonThe gas valve controls gas flow to the burners. If it's stuck, failing, or not receiving the signal to open, gas won't reach the burners for ignition.
Symptoms:
- Ignitor glows but you don't hear gas flow
- No smell of gas when furnace tries to start
- Clicking sound but no ignition
- Error code indicating gas valve or pressure issue
DIY Fix?
No - Call a Pro
Typical Cost
Call for quote
Likelihood
Common
Pro Tip: Never attempt to repair or adjust a gas valve yourself. This requires professional diagnosis and potentially gas pressure testing.
Clogged Burners
ModerateDust, rust, debris, or even insects can clog the small holes (ports) in your furnace burners, preventing gas from flowing properly for ignition.
Symptoms:
- Delayed ignition (loud "boom" or "pop" when starting)
- Uneven or yellow flame instead of blue
- Some burners light but others don't
- Weak flame that struggles to stay lit
DIY Fix?
No - Call a Pro
Typical Cost
Call for quote
Likelihood
Moderate
Pro Tip: Delayed ignition is dangerous—gas builds up before igniting, causing a small explosion. Get this addressed immediately.
Thermocouple or Thermopile Failure
Common (older furnaces)Older furnaces with standing pilot lights use a thermocouple to sense if the pilot is lit. If it fails, it won't allow the main burners to ignite as a safety measure.
Symptoms:
- Pilot light won't stay lit
- Pilot stays lit but main burners won't ignite
- You have to hold the pilot button longer than 60 seconds
DIY Fix?
No - Call a Pro
Typical Cost
Call for quote
Likelihood
Common (older furnaces)
Pro Tip: If your furnace is old enough to have a standing pilot light, it may be time to consider replacement with a more efficient model.
Control Board Malfunction
Less CommonThe control board is the "brain" of your furnace, coordinating the ignition sequence. If it fails, it may not send signals to the ignitor or gas valve.
Symptoms:
- No response at all when thermostat calls for heat
- Random error codes or constantly changing codes
- Some components work but not others
- Furnace worked fine, then stopped completely
DIY Fix?
No - Call a Pro
Typical Cost
Call for quote
Likelihood
Less Common
Pro Tip: Control board issues are often misdiagnosed. A professional should rule out simpler causes first.
Airflow Restrictions (Dirty Filter)
Very CommonA severely clogged air filter restricts airflow, causing the furnace to overheat and trigger the high-limit safety switch, which prevents ignition.
Symptoms:
- Furnace starts, runs briefly, then shuts off
- Furnace won't start at all after overheating
- You can't remember when you last changed the filter
- Visible dust/debris on filter
DIY Fix?
Yes
Typical Cost
Filter cost varies
Likelihood
Very Common
Pro Tip: This is the easiest fix and should be your first check. A dirty filter causes countless furnace problems.
Quick Diagnosis: What Do You See/Hear?
If ignitor glows but burners don't light:
→ Gas valve issue OR clogged burners
If burners light then go out in 3-10 seconds:
→ Dirty flame sensor (most likely)
If ignitor doesn't glow at all:
→ Faulty ignitor OR control board issue
If you hear clicking but nothing happens:
→ Ignitor problem OR gas supply issue
If furnace runs briefly then shuts off:
→ Dirty filter OR flame sensor OR overheating
Frequently Asked Questions
Why is my gas furnace clicking but not igniting?
If you hear clicking (the ignitor trying to spark) but no ignition, the most common causes are: a cracked or worn ignitor that doesn't get hot enough, a dirty flame sensor shutting off gas before it can light, or gas valve issues preventing gas flow. The clicking indicates the system is trying to ignite, so the control board is working.
How do I know if my furnace ignitor is bad?
Signs of a bad ignitor include: furnace won't start at all, you see the ignitor glow but burners don't light, the ignitor glows orange instead of bright white/yellow, or visible cracks in the ignitor element. Ignitors typically last 3-5 years and are a common replacement part.
Can I replace a furnace ignitor myself?
While technically possible for experienced DIYers, we don't recommend it. Ignitor replacement involves working near gas lines and electrical components. A mistake can cause gas leaks, fire, or damage to your furnace. Professional replacement ensures safety and proper operation.
Why does my furnace ignite then shut off immediately?
This is usually a dirty flame sensor. The sensor can't detect the flame properly, so the control board shuts off gas as a safety measure. Other causes include a weak ignitor, incorrect gas pressure, or a cracked heat exchanger (which is a serious safety issue requiring immediate professional attention).
How much does it cost to fix a furnace that won't ignite?
Repair costs vary significantly depending on the cause and your specific system. Simple fixes like flame sensor cleaning are less expensive, while component replacements cost more. We provide free diagnostic assessments and transparent upfront pricing before any work begins—call (908) 249-9701 for a personalized quote.
Can a dirty air filter cause ignition problems?
Yes! A severely clogged air filter restricts airflow, causing the furnace to overheat. This triggers the high-limit switch, which shuts down the furnace as a safety measure. The furnace may attempt to restart and fail repeatedly. Always check your filter first—it's free to inspect and cheap to replace.