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A furnace flame sensor is a very small but very critical component. It detects whether the flame is actually present when the furnace starts. If the flame is absent, it shuts the system down and prevents major breakdowns. When this sensor gets faulty, the result is that the furnace keeps working, gets overheated, and can create bigger issues, including breakdowns and safety risks. The good news is that you can keep your furnace at the best temperature with a little care and regular maintenance. If you need professional HVAC service in Pennsylvania, you can contact Lehigh HVAC for a quick arrival.
You rarely think about the furnace flame sensor until it starts acting up. This little safety component deserves the same attention and care as the rest of the furnace components. This sensor’s job is very simple: it detects whether the flame is actually present when the furnace trips on. If there is no flame, it sends the signal to the control board to immediately turn the system off. No matter if you turn the furnace on again, the same thing keeps happening unless you fix the primary cause. This is just a brief intro to the role a flame sensor plays in a furnace. The actual role might vary slightly depending on the brand and the make and model of the heating appliance.
There are two main types of furnace flame sensors, briefly summarized below.
This is the tiny metallic rod that sits directly in the burner pathway. When the furnace starts, a current flows in the pathway, which is detected by this rod. The rod then sends signals to the control board to continue the heating process. A slight soot or build-up on this sensor blocks its capacity to detect current, and that’s why the furnace gets blocked frequently. If your furnace lights up and then immediately turns off, a dirty rod flame sensor could be the cause.
It differs from the rod flame sensor in both structure and function. First, the rod flame sensor is connected to the furnace through a single wire, while the hot surface sensor is connected through two wires. Second, the rod sensor detects only the flame, while the hot surface sensor detects the flame and ignites the fuel. That’s why, when the hot surface sensor is faulty, the furnace doesn’t light up at all, while it lights up and then turns off if the hot surface sensor is defective.
Fortunately, a furnace shows warning signs when a specific component is not doing its job correctly. Here are the signs you may notice when the flame sensor is faulty.
The furnace turns on and then shuts off after a few seconds
The furnace does not light up at all
The furnace short cycles repeatedly without heating the space
Visible soot or corrosion builds up on the sensor rods
The yellow control board light flashes an error code indicating flame loss
Cracks or visible damage appear on the porcelain insulation around the sensor
The burners ignite normally but extinguish almost immediately afterward
Unusual clicking sounds repeat as the igniter repeatedly attempts to light
Follow the step-by-step guide by Lehigh HVAC to sort it out safely.
Turn off the power and gas supply to the furnace for safety
Locate the flame sensor near the burner assembly
Unscrew and carefully remove the sensor rod
Gently clean the metal rod using a fine emery cloth or steel wool to remove soot and corrosion
Wipe down the sensor with a clean cloth to remove any remaining debris
Reinstall the sensor firmly and secure the mounting screw
Restore gas and power supply, then test the furnace operation
When the DIY yields no results or when you suspect risks in DIY, call a professional. If you are anywhere in Pennsylvania, including Allentown, Bethlehem, Philadelphia, or Carbon County, Lehigh HVAC is the one-stop solution for all your concerns. Book our residential furnace repair service or commercial furnace repair service, depending on your property type, and get all your furnace issues fixed in a single visit. Discuss more at 484-961-7044.
