Furnace Flame Sensor Faults: All You Need to Know

Does your gas furnace shut down shortly after you turn it on? Does this happen several times before the furnace shuts down completely? If that is the case, you may be dealing with a furnace flame sensor problem.

Flame sensors are simple devices designed to confirm the gas valve is only open when there is a fire present, preventing harmful gas buildup. These sensors are typically found in modern furnaces, and other gas-fired heating systems.

Fortunately, problems with flame sensors are a common occurrence among homeowners, and don’t often require complex heating repair. The following brief guide can help you determine if your furnace flame sensor is dirty or faulty, and whether you need expert HVAC assistance in Murrieta.

Do flame sensors wear out?

Flame sensors wear over time. However, often the problem is a dirty sensor, rather than a broken one. Since these sensors are designed to have a low tolerance for reading variations, they might misread from the slightest carbon coating. As most furnaces are located in dusty areas, such as basements or laundry rooms, the airborne particles can easily stick to the flame sensor, burn and cause carbon buildup.

What happens when a flame sensor goes bad?

If cleaning the sensor doesn’t restore your furnace to working condition, a replacement may be in order. If this happens, it is advisable to contact a professional HVAC service provider to inspect your furnace and resolve the issue.

Without a flame sensor, your furnace will keep emitting gas even when there’s nothing to ignite it, resulting in hazardous gas buildup. If your flame sensor is dirty or faulty, it will automatically shut down the heating unit if it doesn’t detect a flame.

Most furnaces will go into a safety ignition lockout when a shutdown happens three times. In addition to being highly inconvenient, continuous tampering with a bad furnace flame sensor can cause issues with other furnace parts and reduce overall efficiency.

How do I know if my furnace flame sensor is bad?

Not sure how to tell if your flame sensor is faulty or just dirty? Follow these guidelines to determine which is true:

  • Switch off the power to your furnace
  • Close the gas valve
  • Take out the mounting screw
  • Carefully pull out the sensor
  • Inspect the sensor: if the insulation is intact but appears sooty, burnt or slightly corroded, it might only require cleaning
  • If the sensor insulation is missing pieces or cracked, a sensor replacement is needed

Replacing a flame sensor is a simple procedure, but it’s advisable to outsource the job to professionals, lowering risk for preventable issues. HVAC experts can also thoroughly inspect your unit to determine any additional problems and prepare your heating system for the upcoming winter season.

What can I use to clean a furnace flame sensor?

It is likely that your flame sensor problem can be fixed with a simple cleaning. The easiest way to do this is to rely on your preferred HVAC technicians in Murrieta to clean the sensor during your regular furnace tune-ups. If you decide to do it yourself, you’ll need:

  • Wrench or driver to remove the 1/4 hex screw (exact tool needed may depend on the type of screw)
  • Light grain sandpaper, steel wool, or a wire brush
  • Clean and dry cloth or paper towel

First, shut off the power to the furnace and locate the sensor in the burner assembly. It is a thin, typically bent, metal rod placed in front of the furnace flame stream. Once located, use the driver or wrench to remove the screw and access the sensor. Next, use the sandpaper, steel wool, or wire brush to rub the rod gently and remove the carbon buildup, making sure you don’t go overboard. Once you’re done cleaning, use the cloth or paper towel to wipe the rod down.

How do you test a flame sensor?

During routine furnace tune-ups and check-ups, your HVAC technicians will troubleshoot your flame sensors to ensure proper performance and safe functionality, as well as prevent more serious damage to the unit. Here’s how it’s done:

Step 1: Unplug the device and locate the sensor. If the unit can’t be unplugged, use the breaker found in the breaker box to switch off the power.
Step 2: Use a driver or wrench to unfasten the screw that holds the flame sensor in place and carefully pull it out.
Step 3: Use a multimeter to show resistance by touching the probes to the white and blue wire ports found on the sensor. The device should detect low resistance.
Step 4: Take the flame sensor and press its open end against a test lamp or a 60 watt bulb. Check the multimeter and see if it shows high resistance.
Step 5: If the multimeter doesn’t detect a resistance discrepancy when placing the open end on a light source, the flame sensor will have to be replaced.

Depend on our skilled heating repair technicians in Murrieta

After being in the HVAC industry for over 45 years, Action Air Conditioning, Heating & Solar has paved its way in the industry. Action is noted as one of the most reliable HVAC and solar companies in its areas. Action’s presence as an industry leader is highlighted by its second consecutive win in The San Diego Union-Tribune Readers’ Poll for “Best HVAC company”.

Whether you want to know about flame sensors or find out if your furnace can function without a filter, our NATE-certified technicians have all the answers. What’s more, you can rely on our expertise-based services across the area, whether you live near Lake Skinner or any of the surrounding neighborhoods. Get in touch with us today and let us handle your heating repair and issues and with professionalism and unrivaled efficiency.


“Complete HVAC system install was done – very professional and clean workmanship. The techs were very communicative during the whole process. I am very pleased with the results . Thanks Action and your team for doing it right the 1st time.”

-John M., Google Review


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