Relighting a pilot is one of the few places where the safety-print on the appliance is the authoritative instruction. Each furnace's lighting sequence is on a label inside the cabinet door or near the gas valve, with safety steps specific to that model. Follow those instructions exactly. If the pilot won't hold a flame after a careful relight per the label, the system needs an HVAC technician. Pilot problems often signal underlying issues (thermocouple, gas valve, flue draft) that involve gas, combustion, and CO risk. We don't write DIY for that.
Why a pilot light goes out
The pilot is a small flame that stays lit so the main burner can light when the thermostat calls for heat. A thermocouple (or flame sensor on newer units) confirms the pilot is lit and tells the gas valve it's safe to deliver gas to the main burner. When the pilot goes out, gas to the burner is automatically shut off as a safety. The pilot itself can go out for several reasons.
1. Failing or dirty thermocouple
The thermocouple sits in the pilot flame and generates a tiny electrical signal that holds the gas valve open. When it's coated with soot, oxidized, or wearing out, the signal weakens and the gas valve shuts off, killing the pilot. This is the most common cause of "pilot won't stay lit" on older standing-pilot furnaces. Cleaning or replacement is a standard HVAC tech repair.
2. Pilot orifice clogged or misaligned
The pilot orifice is a small gas opening. Dust, spider webs, or rust flakes can partially clog it. The result is a weak yellow flame that can't keep the thermocouple hot enough. Sometimes a tiny gust shifts the pilot tube and the flame stops impinging on the thermocouple tip. Both require the cover off the burner area, which is technician work.
3. Draft from a leaky cabinet or flue
If the furnace cabinet panel isn't fully seated, or if the flue has a problem that's pulling combustion air the wrong way, the pilot flame can be blown around or extinguished. Repeated pilot failure after relighting is a flag.
4. Gas supply issue
Low gas pressure, a partially closed shutoff, a stuck gas valve, or work being done somewhere else on the gas system can starve the pilot. If other gas appliances in the house are also having trouble, the utility or service plumber is the call.
5. End of life on an electronic ignition system
Most newer furnaces don't have standing pilots; they use electronic ignition (hot surface ignitor or spark). When these fail, the symptom looks similar: no burner fire, no heat. Hot surface ignitors have a typical life of 3 to 5 years. Replacement is a technician task.
What to do
- Follow the relighting instructions on the cabinet label. Every furnace has them. They include critical timing for waiting after gas has been off (typically 5 minutes for gas to clear before relighting).
- If the pilot won't light at all, turn the gas control to "Off," wait 5 minutes, and either try again per the label or stop and call a technician.
- If the pilot lights but won't stay lit after you release the manual button, the thermocouple is almost certainly the issue. Call a technician.
- If you smell gas at any point, stop. Turn the gas off at the appliance shutoff valve if you can do it safely. Leave the house. Call your gas utility from outside.
- If a CO alarm sounds at any point, get everyone outside and call 911.
What not to do
- Don't take the cover off the burner area to inspect the thermocouple yourself. Gas, combustion, and electrical risk in one spot. The label on the cabinet is the only DIY guidance.
- Don't keep trying to light the pilot repeatedly. Each failed attempt releases unburned gas into the cabinet. Wait the recommended time between attempts.
- Don't tape, hold open, or modify safety switches to "force" a burner to fire. They exist to prevent fires and CO release.
- Don't use the gas range or oven to heat the house if the furnace is down. That's a known CO risk.
When to consider whether the furnace is past its life
Standing-pilot furnaces are typically older units, often 20 to 30 years old or more. If you're calling for thermocouple replacement on a furnace that's also showing rust, sediment in the heat exchanger, or yellow burner flames, the technician's evaluation may favor replacement over repeated repairs. Modern condensing furnaces are 90 to 97% AFUE compared to 60 to 70% on older units, so the energy savings can pay back replacement over a few heating seasons.
If the furnace has been off for the season
Pilots sometimes go out during summer storage or after a power blip. If your furnace has a standing pilot, this isn't necessarily a sign of a failed component. Follow the cabinet label to relight at the start of heating season. If it holds, you're fine. If it doesn't hold past the first heating cycle, the thermocouple or another component needs attention.
Good maintenance rhythm
- Yearly in fall: professional inspection. The tech checks the pilot, thermocouple, flame sensor, gas pressure, and combustion. Most pilot issues get caught during this visit.
- Monthly: press the test button on every smoke and CO detector.
- Before each heating season: relight the standing pilot per the cabinet label if it went out during storage.
- If the pilot won't hold after a careful relight: call an HVAC tech before running the furnace.
- If you smell gas at any point: leave the house and call the utility.
- If the CO alarm sounds at any point: leave and call 911.
- Track furnace age. Past 20 years, plan for replacement rather than chase repeated repairs.