Common reasons include a nearby draft, dirt buildup, or a malfunctioning thermocouple. Another pilot light drawback is energy waste. Since the pilot light needs to remain active, your furnace is always consuming some gas. It's not a lot—usually just a few therms of gas a month–but that obviously adds up over time.
A mispositioned or undersized pilot flame (too far from the burner) usually causes this. A dirty of rusty burner can cause this also (if the burner gas is restricted near the pilot. Then the flame has to spread from the front to the back of the burner and from burner to burner without delays or it will backflash.
Faulty thermocouple
If the gas furnace pilot light won't stay lit, the thermocouple will shut down the gas valve, so gas cannot be released into the home. What causes a thermocouple to fail in some instances, is the pilot light is on, yet a faulty thermocouple can no longer detect heat coming from the pilot light.
The flame will go out if you release the knob before it is hot. The thermocouple can become dirty and caked with grit, preventing it from heating up and detecting that the pilot is lit. As a result, the pilot will always go out after you release the reset button.
Troubleshooting: If the pilot light won't stay lit, the thermocouple might need cleaning or replacement. To clean it, gently remove any soot or debris with a soft cloth. If cleaning doesn't work, you may need to replace the thermocouple, which is best done by a professional.
So you've checked your pilot light and even reset it, but still, it wont stay lit. This can be an indication that your fireplace thermocouple is either bad or worn out. However, if your pilot is lit but the gas fireplace isn't, then there's probably a problem with your thermopile.
The most common cause of this is dirty or blocked burners, which is the part that you can easily take off to clean.
Common reasons include a nearby draft, dirt buildup, or a malfunctioning thermocouple. Another pilot light drawback is energy waste. Since the pilot light needs to remain active, your furnace is always consuming some gas. It's not a lot—usually just a few therms of gas a month–but that obviously adds up over time.
There's really just one telltale sign of letting you know when the thermocouple isn't working, and that's when the pilot light won't stay lit. If you experience this problem, it's important to have your thermocouple assessed as soon as possible.
If you try to relight a pilot light and the thermocouple is bent in a way that causes it to not make contact with the pilot light, the flame will instantly go out when you release the pilot control knob since the probe won't detect the heat from the flame.
For the burners to ignite, they need airflow. The airflow into the furnace comes from the blower fan that draws air through the return air ducts. But if this airflow is choked off because of a clogged furnace filter or faulty blower, the burners may not be able to stay on for long.
Possible causes include a broken thermostat; the thermostat being set too low; an out pilot light; a blown fuse or circuit breaker; a closed gas valve; or electronic ignition problems.
Possible causes are a bad blower motor, a clogged air filter, blocked or restricted registers, or under-sized ductwork.
If your gas fireplace pilot light keeps going out, you may have a problem with faulty pilot controls. The pilot control failing due to age or a mechanical issue can cause your pilot light to keep going out and prevent it from being relit.
It's easy to replace, and a new one costs $5 to $10. Rather than trying to unfasten the thermocouple inside the water heater's burning chamber, it's easier to remove the whole burner and thermocouple assembly.
The thermocouple is a safety device designed to shut off the gas valve if the pilot light goes out. Although, if your thermocouple is old, worn, damaged, wrongly positioned, or malfunctioning, it could have wrongly shut off the gas valve and prevented your pilot light from staying lit.
For those who prefer not to attempt a DIY repair, hiring an HVAC technician is a safer option. The cost for a professional furnace thermocouple replacement usually ranges from $80 to $150, depending on your location and the specific HVAC company. This price includes both the part and labor.
Once you have replaced the thermocouple, turn on the furnace and check to see if the pilot light stays lit. If your pilot light doesn't stay lit, it may be due to something blocking the thermocouple. You can try cleaning or replacing it, but if that doesn't work, you may need to call a professional.
Thermocouple Malfunction
It is positioned very close to the flame, and when lit, the pilot light tells the thermocouple to keep the gas line open. A thermocouple that is dirty or malfunctioning will often shut off your gas supply in error, causing the pilot light to go out and your furnace to stop working.
If your pilot light won't stay alight, it might be the thermocouple. Sometimes dust buildup collects on the thermocouple and blocks the sensor. Try cleaning the thermocouple and then relighting the pilot light. If it continues to go out, the thermocouple might need to be replaced.
Dirty flame sensor
If this sensor is dirty, the dirt can cause the sensor to operate intermittently. When this happens, your furnace will most likely light, but because the sensor can't properly detect if a flame is present. The sensor may signal the ignition cycle to end, which your furnace won't stay lit.
The most like culprits are likely tied to some kind of problem with your setup. Anything from issues with your pilot light, clogged ports on burners, damage to heating components, and thermocouple/thermopile malfunctions can be to blame.