Pilot Light Won't Stay Lit
A pilot light that won't stay lit is one of the most common furnace and water heater problems. The good news is that most causes are simple and fixable. The important thing is to understand what you're dealing with before you start poking around gas equipment.
What to Check Before You Call Someone
Make sure there's no gas smell
Before doing anything else, check for the smell of gas (rotten egg odor). If you smell gas strongly, this is NOT a DIY situation.
Fix: If you smell gas: don't flip any switches or use your phone. Leave the house immediately, then call your gas company or 911 from outside. If there's no gas smell, continue with the steps below.
Check the thermocouple position
The thermocouple is a small metal rod that sits in the pilot flame. It senses whether the pilot is lit and keeps the gas valve open. If the thermocouple has shifted away from the flame (even by a quarter inch), it can't sense the heat and shuts off the gas as a safety measure.
Fix: Gently bend the thermocouple so the tip sits directly in the pilot flame... it should be engulfed by the flame, not just near it. Relight the pilot and hold the button for 30-60 seconds to let the thermocouple heat up fully before releasing.
Clean the thermocouple
Carbon buildup, dust, or oxidation on the thermocouple tip insulates it from the flame, making it think the pilot is out when it's actually burning.
Fix: Turn off the gas and let everything cool. Lightly sand the thermocouple tip with fine sandpaper or emery cloth until it's shiny. This removes the insulating layer and restores proper heat sensing. Costs nothing and takes 2 minutes.
Check for drafts blowing out the pilot
If the furnace or water heater is near an exterior door, window, or dryer vent, drafts can blow out the pilot light. This is more common than people realize, especially when the furnace is in a garage or near a door that gets opened frequently.
Fix: Identify and block the draft source. If it's coming from the combustion air intake, make sure the intake vent isn't positioned where wind hits it directly. The pilot should be shielded from cross-drafts by the burner assembly.
Replace the thermocouple
If cleaning and repositioning didn't work, the thermocouple has likely reached end of life. They last 5-10 years and are a wear item. This is the most common fix for a pilot that won't stay lit.
Fix: A universal thermocouple costs $10-$15 at any hardware store. Turn off the gas, unscrew the old thermocouple from the gas valve (one nut), detach it from the pilot bracket, install the new one, and relight. The whole job takes 15-20 minutes. If you're not comfortable working near gas connections, a service call costs $100-$200.
Check the gas valve itself
If a new thermocouple doesn't fix it, the gas valve may be faulty. The electromagnet inside the valve that holds the gas open (triggered by the thermocouple signal) can weaken or fail over time.
Fix: Gas valve replacement is a professional job. The valve costs $100-$300 for the part, and labor is $100-$200. On a furnace over 15 years old, a failed gas valve is often the tipping point for full replacement... the rest of the system is aging too.
When It's Time to Replace
If the gas valve has failed on a furnace that's over 15-18 years old, replacement of the whole furnace makes more sense than a $300-$500 gas valve repair. At that age, other expensive components (heat exchanger, blower motor) are likely to fail within a few years. A pilot light issue on a newer furnace (under 10 years) is almost always a cheap thermocouple fix.
Read our full Furnace replacement guide →When to Call a Professional
Call a professional if you smell gas at any point, if replacing the thermocouple doesn't fix the problem, or if you're not comfortable working around gas connections. Also call if your furnace has a sealed combustion chamber that you can't easily access... modern high-efficiency furnaces have electronic ignition rather than standing pilot lights, and their ignition systems are more complex.
This guide is for informational purposes. For gas leaks, electrical issues, or emergencies, call a licensed professional immediately.