What causes a furnace to light then go out?

If the flame sensor is dirty or has become corroded, the flame won’t register and will turn off the gas valve, which, in turn, causes your system to shut down. You will need to contact a professional to check this, especially if your furnace is cycling on and off too quickly.

Why does my pilot light keep cutting off?

A faulty, misaligned, dirty, or damaged thermocouple is often the culprit behind your pilot light going out. The thermocouple is an automatic safety shutoff device that prevents the gas valve from opening if the pilot light is out.

What does it mean when your furnace won’t stay lit?

Probably the most common issue when dealing with a furnace that will not stay lit or ignite consistently is a dirty flame sensor. The flame sensor acts as a communication vehicle with the circuit board and tells it when a flame is lit inside the furnace.

How do you fix a pilot that won’t stay lit?

Quote from the video:
Quote from Youtube video: I don't advise using a torch to do this but this is all I have at the moment so once you hold the tube down well first of all we turn this on. -. So I'll turn the gas on.

Why does my furnace only run for a few seconds?

A common reason a furnace short cycles is because the heat exchanger, the part that heats your air, is getting too hot due to a lack of airflow. Once the heat exchanger overheats, it trips the high limit switch, which shuts the furnace down to protect it.

Why is my gas furnace short cycling?

A dirty air filter is the most common cause of short-cycling. If your air filter is clogged with dirt, it won’t allow proper air flow to the heat exchanger. That can cause the furnace to overheat and forces the high limit switch to ride to the rescue by shutting the furnace off.

How do I know if my thermocouple is broken?

If you can’t get the flame to light at all, and you’re sure the gas is on, there’s probably an obstruction in the pilot tube. If the flame lights and goes out when you release the gas control knob after holding it in for the recommended 20 to 30 seconds, that’s the sign of a thermocouple malfunction.

How much does it cost to replace a thermocouple on a furnace?

$150 to $250

Replacing a thermocouple costs $150 to $250, mostly due to labor time. On older models without an ignitor, it checks to make sure your pilot light is lit. If it’s not, it’ll shut the gas off.

How much does it cost to replace a thermocouple?

1 cause of pilot lights that won’t stay lit is a worn-out thermocouple. 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.

How do you clean a thermocouple on a furnace?

Quote from the video:
Quote from Youtube video: Use a soft abrasive pad or emery cloth to gently clean the outside of the thermocouple. Be careful not to scratch the coating. Next insert the thermocouple into the pilot assembly.

How do I clean a dirty pilot tube?

Quote from the video:
Quote from Youtube video: The easiest way to clean a pilot assembly is the only way operating pump once or twice will easily clear the ash from the orifice. Inside the pilot assembly.

What happens if a thermocouple fails?

When a thermocouple fails, the pilot light won’t stay lighted. A burned out or broken thermocouple should be replaced.

Can you clean a thermocouple?

Yes, you can clean a thermocouple, but the task must be done with a light touch. Remove the thermocouple from the process assembly, using a steel wool or emery cloth, lightly clean the tip removing any build up. Cleaning a thermocouple is not a fix for an issue, such as inaccurate readings.

How often do thermocouples go bad?

Thermocouples can last up to 10 years if properly installed and maintained, while furnaces can last 15-20 years, although either may wear out sooner in multifamily units.

What is the most common thermocouple failure mode?

The most common failure mode for thermocouples is for them to fail open, this is known as thermocouple burnout. Burnout can result from a variety of reasons including the thermocouple experiencing a temperature in excess of that for which it was designed, metal fatigue, and oxidation.

How does a thermocouple wear out?

The presence of oxygen causes a reaction in the thermocouple that can wear down the part and make the wires become thinner and more brittle. Oxidation can be caused by a small amount of oxygen making its way into a sealed environment and reacting with the pure metal of the thermocouple.

What are the sources of errors in thermocouple reading?

Because a thermocouple measures temperature differentials, any temperature fluctuations around the reference junction (cold junction), which has the known temperature, result in an erroneous temperature reading. Solution: Make sure no fans or other sources of cooling or heating are located near the reference junction.

What does thermocouple look like?

The thermocouple is made up of a tube, a bracket, and wires. The tube sits above the bracket, a nut holds the bracket and wires in place, and underneath the bracket, you will see the copper lead wires that connect to the gas valve on the furnace.

How can thermocouple error be reduced?

Simply by switching to a Type E thermocouple you can cut your error to ±1.7°C/3°F. That is a 22% decrease in error. Using a “Special Limits of Error” SLE grade Type E probe further reduces the error further to ±1°C/1.8°F.