Gas Furnace Not Igniting — Igniter, Flame Sensor, Gas Valve Diagnosis
A gas furnace that won't ignite — the inducer runs, the igniter glows (or tries to), but no flame appears — points to one of three main components: the hot surface igniter, the flame sensor, or the gas valve. The most common cause by far is a dirty flame sensor — a 10-minute free fix. Work through each component in this order before calling a technician.
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Common Symptoms
- Furnace inducer starts and igniter glows but no flame appears
- Furnace fires briefly (2–3 seconds) then shuts off with a lockout
- Error code for ignition failure (Carrier code 14, Lennox code 33)
- Strong gas smell near the furnace
- No glow from the igniter at all during the startup sequence
Most Likely Causes
- 1
Dirty Flame Sensor (Most Common)
The flame sensor is a thin metal rod in the burner that detects whether flame is present. Over time, it develops a coat of oxidation that insulates it — the furnace fires for 2–3 seconds, the sensor can't confirm flame, and the gas valve closes. Cleaning the flame sensor with fine steel wool is the #1 furnace DIY repair — free and takes 10 minutes.
- 2
Cracked or Failed Hot Surface Igniter
The hot surface igniter (a silicon carbide or silicon nitride rod) heats to 1,800°F to ignite the gas. These are fragile — they crack from vibration, thermal shock, or age. A cracked igniter may glow partially but not reach ignition temperature. Test with a multimeter: resistance should be 40–75 ohms for most igniters; open circuit = failed.
- 3
Gas Valve Not Opening
If the igniter glows correctly but no gas flows, the gas valve has failed or isn't receiving the 24V signal to open. First confirm gas supply (other gas appliances work, gas valve knob is ON). Test for 24V AC at the gas valve terminals when the igniter reaches temperature.
- 4
Pressure Switch Preventing Ignition
If the pressure switch isn't closing, the furnace's IFC board never advances to the ignition step. The inducer will run but no ignition attempt will occur — the igniter never glows. Check the pressure switch circuit first if the igniter never activates.
- 5
Cracked or Blocked Burner
Corroded or blocked burner ports prevent gas from flowing correctly. You may see a partial or delayed ignition — some burners light and others don't, or a delayed 'puff' when gas finally reaches the igniter. Inspect the burners for rust, spider webs (common in warm climates), or debris.
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Quick DIY Checks
If you smell gas strongly at any point, do not attempt ignition. Turn off the furnace and main gas supply valve, open windows, leave the building, and call your gas company.
Do not touch the hot surface igniter with bare hands. Skin oils contaminate the silicon carbide and cause premature failure. Handle with a clean rag or nitrile gloves.
- 1Observe one full startup attempt through the furnace observation window. Note exactly what happens: does the inducer start? Does the igniter glow? Does gas ignite? Does the flame hold? This tells you which step in the sequence is failing.
- 2Locate and clean the flame sensor. It's a single thin metal rod with a porcelain base, mounted in the burner area with one wire connected. Remove the mounting screw, pull it out, and rub the metal rod lightly with fine (0000) steel wool until clean and shiny. Reinstall and test.
- 3Test the hot surface igniter through the observation window — it should glow bright orange within 60 seconds of startup. If it glows but no gas lights, the issue is downstream (gas valve or burner). If it doesn't glow at all, test resistance: should read 40–75 ohms. Open = failed igniter.
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Try Pro — $7.99/mo- 4Confirm gas supply. Turn off the furnace and check other gas appliances. If they work, gas supply is fine. On the furnace, confirm the gas shutoff valve near the unit is fully open (handle parallel to the pipe on a ball valve).
- 5Check the 24V signal to the gas valve. With a multimeter set to AC volts, probe the gas valve terminals when the furnace should be trying to open the valve. Should read 24–28V AC. No voltage = control board or wiring fault. 24V present but valve doesn't open = failed gas valve.
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Repair vs Replace
Ignition problems are almost always worth repairing. Flame sensor cleaning is free; igniter replacement is $30–$80 and takes 20 minutes. Gas valve replacement runs $150–$350 with parts. Only consider replacement if the heat exchanger has also cracked — that changes the economics entirely.
Est. Repair Cost
$0 (clean flame sensor); $30–$80 (igniter); $150–$350 (gas valve)
Est. Replacement Cost
$2,500–$5,000 for a new furnace
Recommended Tools & Parts
- Buy on Amazon →
Universal Hot Surface Igniter (Silicon Nitride)
Universal 80V or 120V replacement igniter for most gas furnaces. Confirm voltage rating from your furnace wiring diagram before ordering.
$20–$45
- Buy on Amazon →
Furnace Flame Sensor (Universal Rod)
Replacement flame sensor rod for gas furnaces. Use if sensor is cracked, heavily corroded, or cleaning doesn't restore function.
$5–$15
- Buy on Amazon →
Digital Multimeter
Required for testing igniter resistance, gas valve voltage, and flame sensor microamp output. An essential tool for any furnace diagnosis.
$15–$30
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Frequently Asked Questions
- How do I clean a furnace flame sensor?
- Locate the flame sensor — a thin metal rod (looks like a spark plug without the tip) mounted with a single screw near the burner flame area, with one wire attached. Remove the screw, pull out the sensor, and lightly rub the metal sensing rod with 0000 steel wool or 400-grit sandpaper until it looks clean and shiny. Reinstall, connect the wire, and test. Do not touch the metal rod with bare hands after cleaning.
- How do I know if my furnace igniter is bad?
- Watch it during a startup attempt through the observation window. A working igniter glows bright orange-white within 60 seconds of startup. If it doesn't glow at all, or glows very dimly, remove it and test with a multimeter — look for 40–75 ohms resistance. An open reading (OL or no continuity) means the element is broken and the igniter needs replacement.
- Furnace lights but goes out after 2–3 seconds — is that the gas valve?
- Almost certainly not — a gas valve either opens or it doesn't. A flame that appears and then extinguishes after 2–3 seconds is the classic dirty flame sensor symptom. The gas lights, but the oxidized sensor can't generate enough microamp signal to confirm flame, so the control board closes the gas valve as a safety measure. Clean the sensor first — it's the right diagnosis the vast majority of the time.