If the pilot lights but goes out as soon as you let go of the gas control knob, the thermocouple is usually the cause: it is worn, loose, or no longer sitting in the flame. If the pilot stays lit for hours or days and then goes out, look at a weak or dirty pilot flame, a draft, or a thermocouple near the end of its life. A pilot that won’t light at all usually means no gas is reaching it.
Standing pilots are found on older furnaces and on many wall and floor furnaces. Most furnaces built in recent decades have electronic ignition and no pilot at all; if yours has none and won’t heat, see furnace not producing heat.
If you smell gas, don’t relight anything. Don’t touch light switches or the thermostat. Leave the house and call the gas utility or 911 from outside.
How the pilot keeps the gas on
The pilot is a small flame that burns all the time and lights the main burners when the thermostat calls for heat. It also heats the tip of the thermocouple, a metal probe that produces a small voltage when it is hot. That voltage holds a safety valve inside the gas control open. When the pilot goes out, the thermocouple cools, the voltage drops and the valve closes, so unburned gas doesn’t keep flowing.
That is why you hold the knob down while lighting: you are holding the valve open by hand until the thermocouple is hot enough to take over. A thermocouple that is worn, cracked or out of position can’t hold it, and the pilot dies the moment you let go.
What the pilot is telling you
| What happens | Likely cause | What it takes |
|---|---|---|
| Lights, goes out as soon as you release the knob | Worn thermocouple, loose connection, or thermocouple out of the flame | Reposition, tighten or replace the thermocouple |
| Stays lit, goes out hours or days later | Weak flame, a marginal thermocouple, or a draft | Pilot cleaning; thermocouple test; find the draft |
| Small, yellow or flickering pilot | Dirty pilot orifice or tube | Pilot assembly cleaning |
| Goes out when the burners or blower start, or a door opens | Draft, a bent pilot shield, a missing burner cover | Cover back on; fix the draft |
| No flame and no hiss of gas when you try | Gas valve or meter off, or a supply problem | Check the valve and other gas appliances; call the utility |
| Pilot hood damaged or corroded | Pilot assembly damage | Pilot assembly replacement |
| Pilot stays lit, burners never come on | Thermostat, gas valve or control problem | Diagnosis by a technician |
Relighting safely
The steps for your furnace are printed on a label on or inside its door; follow them. On most standing-pilot furnaces they run like this:
- Set the thermostat to Off or its lowest setting.
- Turn the gas control knob to Off and wait as long as the label says (often five minutes) for any gas to clear.
- Smell around the furnace, down near the floor too. If you smell gas, stop, leave and call from outside.
- Turn the knob to Pilot, press and hold it, and light the pilot with the built-in igniter button or a long lighter, as the label directs.
- Keep holding for about a minute so the thermocouple heats up, then release slowly.
- If the pilot stays lit, turn the knob to On, put the cover back and set the thermostat.
While it burns, look at the flame: it should be steady, mostly blue, and wrap around the tip of the thermocouple. Check that the gas valve on the supply pipe is open (handle in line with the pipe) and that no draft blows across the burner area. If the pilot goes out again after one or two tries, stop. Repeated attempts won’t fix a thermocouple, and each one lets out a little gas.
When to call a technician
- The pilot won’t light at all.
- It won’t stay lit after relighting.
- You smell gas (leave first, call from outside).
- The thermocouple needs replacing or repositioning.
- You are unsure about the relighting steps.
- You see soot or scorch marks, or the main burner flame is yellow; see cracked heat exchanger.
A thermocouple is a small part and usually a quick furnace repair. A furnace with a standing pilot is also usually an old furnace, so it is worth having the heat exchanger and venting checked on the same visit. Whether to keep repairing it or move to a modern furnace with electronic ignition is your call; we explain the trade-offs and you decide. See furnace installation.
Local note
Wind off the ocean is part of Daly City weather, and many furnaces here sit in ground-floor garages. Gusts through the gaps around a garage door can disturb a weak pilot, especially when the burner compartment cover is off. If your mid-century home still has an older furnace, our mid-century home HVAC guide covers what to expect from it.