Flame Sensors

The flame sensor is a small metal rod inside your furnace burner compartment that confirms the gas burner is actually lit. When it gets dirty or fails, your furnace lights for 3-7 seconds then shut...

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How to Choose the Right Furnace Flame Sensor

The flame sensor is a small metal rod that verifies gas is actually burning after the igniter lights the burners. It works by detecting the electrical conductivity of the flame — when gas burns, it creates a small microamp current that the control board monitors. If the board does not detect this current within a few seconds of opening the gas valve, it shuts everything down as a safety measure. A dirty or failed flame sensor is one of the most common causes of furnace lockout.

How Flame Sensors Work

The flame sensor sits in the burner flame path and conducts a tiny DC current (typically 1-6 microamps) through the flame to ground. The control board monitors this current to confirm combustion is occurring. Over time, the sensor rod develops an oxide coating that insulates it, reducing the current below the board threshold and causing a "no flame detected" lockout even though the flame is burning normally.

Cleaning vs Replacing

A dirty flame sensor can often be restored by gently cleaning the rod with fine emery cloth or steel wool to remove the oxide buildup. However, if cleaning provides only temporary relief (the sensor dirties again quickly), or if the rod shows pitting, corrosion, or cracks in the ceramic insulator, replacement is the better solution. At the low cost of a flame sensor, many technicians replace rather than clean as standard practice.

Sensor Shapes and Mounting

Flame sensors come in various shapes to fit different burner configurations: straight rods, L-shaped or bent rods, and custom angles. The mounting bracket, rod length, and bend angle must match your furnace to position the sensor correctly in the flame path. An incorrectly positioned sensor will not detect the flame reliably, even if it is new and clean.

Signs Your Flame Sensor Needs Attention

The classic symptom is the furnace lighting briefly (3-7 seconds) and then shutting off, repeating this cycle three times before locking out. The burners ignite, the flame appears, but the control board does not detect it and closes the gas valve. This "light and lockout" pattern is the hallmark of a flame sensor issue. The diagnostic LED typically shows a flame failure code.

Finding Your Replacement

Check the part number on your existing sensor or your furnace manual. Use our Look Up by Model Number tool above to find the exact OEM replacement. Every order placed before 3 PM CST ships same day from National HVAC Parts.

Frequently Asked Questions

How often should I clean my flame sensor?

As part of annual furnace maintenance, the flame sensor should be removed and gently cleaned with fine emery cloth. If you find yourself cleaning it more than once per season, the sensor is likely worn and should be replaced. A new sensor is inexpensive insurance against mid-winter lockouts.

Can I use a universal flame sensor?

Universal flame sensors work if the rod length, mounting bracket, and wire connector match your furnace. The sensing rod must be positioned correctly in the flame path. Many universal sensors include multiple mounting brackets and adjustable rod angles. Always verify physical compatibility before installing.

What microamp reading indicates a good flame sensor?

A healthy flame sensor typically reads 2-6 microamps when measured with a multimeter in the DC microamp range in series with the sensor wire. Below 1.5 microamps, the sensor may cause intermittent lockouts. Below 0.5 microamps, lockout is almost certain. If cleaning does not restore the reading above 2 microamps, replace the sensor.

Why does my furnace light then shut off after a few seconds?

This is the classic flame sensor symptom. The furnace ignites normally but the control board cannot verify the flame through the dirty or failed sensor, so it closes the gas valve as a safety measure. After three failed attempts, it locks out. Clean or replace the flame sensor to resolve this. Less common causes include a cracked sensor insulator or a bad ground connection on the burner assembly.

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