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Diagnostic guide

Furnace Pilot Won't Light

Older standing-pilot furnaces need gas flow, thermocouple signal, and clean pilot orifice — electronic ignition systems differ; know your type before troubleshooting.

System
Heating
Causes covered
7
You can check
1 without a technician

Stop and read this first

  • If you smell unburned gas after failed lighting attempts, shut gas and ventilate.
  • Do not bypass safety valves to keep heat on.

Work through these in order

Ordered from most to least likely. Working top down means you rule out the cheap, common causes before paying anyone to look at the expensive, rare ones.

  1. Thermocouple failed

    CommonNeeds a pro
    How to check
    Pilot lights then goes out on release.
    The fix
    Replace thermocouple ($150 to $250).

  2. Clogged pilot orifice

    CommonNeeds a pro
    How to check
    Weak blue flame or no flame.
    The fix
    Clean orifice with fine wire; tech service.

  3. Gas valve off or supply issue

    CommonYou can check
    How to check
    No gas to other appliances.
    The fix
    Open valve; check utility outage.

  4. Dirty flame sensor (intermittent pilot/IID)

    CommonNeeds a pro
    How to check
    Flame present; furnace does not stay lit.
    The fix
    Clean sensor rod.

  5. Draft blowing pilot out

    PossibleNeeds a pro
    How to check
    Flame unstable near vent.
    The fix
    Check vent cap and combustion air.

  6. Faulty gas valve

    PossibleNeeds a pro
    How to check
    No click; no gas to pilot.
    The fix
    Replace valve ($300 to $600).

  7. Converted to electronic ignition needed

    UncommonNeeds a pro
    How to check
    Pilot obsolete; parts scarce.
    The fix
    Upgrade ignition module or furnace.

Direct answer

Standing pilot won't stay lit — thermocouple or dirty orifice on older units; newer IID systems differ. If gas smell builds, ventilate and call.

Severity and urgency

Fix within days

No heat in freezing weather is urgent for pipes.

Cost of ignoring it

Frozen pipes $500–$5,000; valve $300–$600 repair.

How a pro will diagnose it

Test thermocouple millivolts, clean pilot, check gas valve ($150–$300).

Prevention and maintenance

  • Annual tune-up
  • Keep intake clear
  • Replace thermocouple proactively at 10 yr
  • Know emergency heat
  • Filter monthly

For trade owners

Pricing this job as a contractor?

Homeowners see national ranges on this page. If you are building the estimate, use the same scope assumptions — then price to burdened labor, overhead recovery and a real margin.

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Common questions

Could thermocouple failed be causing this?
Pilot lights then goes out on release. Replace thermocouple ($150 to $250).
Could clogged pilot orifice be causing this?
Weak blue flame or no flame. Clean orifice with fine wire; tech service.
Could gas valve off or supply issue be causing this?
No gas to other appliances. Open valve; check utility outage.
Could dirty flame sensor (intermittent pilot/iid) be causing this?
Flame present; furnace does not stay lit. Clean sensor rod.
What is the most common cause of furnace pilot won't light?
Follow relight label exactly.
Can I fix furnace pilot won't light myself?
Some checks are DIY-safe (shutoffs, filters, simple adjustments). Licensed HVAC technician work is required when gas, sewer, refrigerant, or main lines are involved.
How much does professional repair for furnace pilot won't light cost?
Thermocouple $150–$250.
How urgent is furnace pilot won't light?
Gas smell after attempts — shut gas.
Should I shut off water or power for furnace pilot won't light?
Shut off the local supply or breaker when leaks, flooding, sparks, or gas odor are present. If unsure, stop using the system and call a pro.
Will furnace pilot won't light damage my home if I wait?
Moisture-related issues can rot framing and grow mold within 48 hours. Mechanical issues often worsen — seized pumps, cracked heat exchangers, or compressor damage cost far more when delayed.
What should I tell the HVAC technician on the phone?
Describe when it started, what changed (weather, remodel, new appliance), what you already tried, and whether multiple fixtures or zones are affected.
Does homeowners insurance cover furnace pilot won't light?
Sudden pipe bursts are often covered; slow leaks, maintenance neglect, and sewer backups frequently are not. Document with photos and read your policy endorsements.
How do I know the quote for furnace pilot won't light is fair?
Get two written quotes with scope, parts, warranty, and permit line items. Compare to regional mid-range pricing — not the cheapest emergency rate.
How can I prevent furnace pilot won't light from coming back?
Follow the prevention steps on this page: maintenance schedules, avoiding grease and wipes in drains, filter changes, and annual tune-ups where applicable.

Once you know what it is

Step one

Know what it should cost

Get the range before you call, so the first number you hear is not the only number you have.

Step two

Decide repair or replace

On equipment near end of life, the repair is usually the first of several.

Step three

Check the quote

Compare against the typical range and confirm the terms before you sign.

Furnace Replacement cost guideRepair or replace calculatorEstimate repair costAnalyze a quote

What the fix costs

Tools for this decision

Other symptoms