Airflow through the furnace comes from the return air ducts and must pass through a filter (which keeps dust and debris from getting into the furnace). This filter will become clogged after one to three months, and unless it's replaced with a clean one, it can cause the burners to go out early.
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.
The flame sensor does exactly what it sounds like, it senses when the flame of the furnace is on and sends a message to the control board to start blowing warm air. If dirt or dust covers the sensor, the flame isn't detected, and the furnace shuts down.
There are many possible reasons why the burner will not stay lit - the gas jet may be blocked, the valve may be set too low, the air flow may be restricted, there may be an issue with the pilot light, and so on.
The flame sensor should not have visible dirt or corrosion on it. If you don't get routine maintenance for the furnace, then it's more likely you will develop these problems. Your technician will otherwise clean off the sensor every year when they perform the maintenance.
Most furnace flame sensors cost less than $20, but if you opt to have a professional HVAC technician make the repair, you can expect to pay between $75 and $250. Need help replacing your flame sensor?
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 common cause of this is dirty or blocked burners, which is the part that you can easily take off to clean.
Your gas fireplace could be turning off for several reasons. 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.
To begin, ensure that all the filters of your furnace are clean. Also, get the filters replaced every few months to ease the burden on your HVAC system and ensure that fresh air comes filtered into your home. Another common cause of your furnace turning on and off after a few minutes is a faulty pressure switch.
The Furnace will only ignite if the thermostat is functional. Double-check that the temperature is set higher than the temperature of the surroundings. If you have a programmable device, check for an error code. Once you've inquired about all these matters, restart your thermostat.
There are multiple reasons why your furnace is short cycling. These could include a dirty filter, blocked vent, or malfunctioning flame sensor. You can switch for a clean furnace filter, or have your HVAC technician resolve a more complicated issue.
Failed Thermocouple Symptoms
The first is signs of contamination on the tube, which can include discoloration, cracks, or pinholes. Next, check the wiring for any signs of wear or corrosion like missing insulation or bare wire.
A dirty or corroded flame sensor can cause your furnace to keep shutting off unexpectedly. The flame sensor detects if the furnace's burner is lit, and if it doesn't register the flame, it will shut off the gas valve to prevent a gas leak.
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.
If the pilot light of your gas heater fails to ignite or stay lit, it may be due to a dirty pilot assembly. Over time, the pilot assembly can accumulate dust, debris, or other contaminants, which can obstruct the flow of gas or impede the proper functioning of the flame.
There is a work-around to waiting. If there is no safety risk (like a gas leak), a furnace can be reset to bypass this wait. Turn off the furnace's power switch and wait 10 seconds before turning it back on.
A heat exchanger replacement is often one of the most costly repairs in a furnace, and the expense can sometimes rival that of installing a new furnace. Typical replacement costs vary widely but can easily reach several thousand dollars, especially if labor costs are factored in.
While most furnaces last for 10 to 20 years, flame sensors have a much shorter lifespan of about five years. Usually, when a flame sensor goes bad, the best route is to replace it outright.