Furnace flame sensor: Guide

What it is

The flame sensor is a metal rod that sits in the burner flame. The control board uses the tiny current that flows through the flame to confirm the gas has lit. If the rod is coated, the board may not sense flame and will shut the gas off again, often a few seconds after ignition.

Several manuals name this as a cause: Carrier lists oxide build-up on the flame sensor, Trane lists a normal flame current range, and Lennox gives a low-signal threshold.

How to choose

Match the part number on your furnace rating plate or in the parts list of your manual. Rod length, bracket shape and connector differ between furnaces.
A universal kit can only work if its bracket and rod shape match the original; compare it to the old part before relying on it.
Our links open search results, not a confirmed fit for your model.

How it is tested

Carrier says to check for oxide build-up on the flame sensor and clean it with fine steel wool. Rheem recommends the sensor be cleaned at the start of each heating season with fine steel wool or a Scotch-Brite pad by a qualified installer, service agency or the gas supplier.
Trane lists a normal flame current of 0.75 to 3.0 microamps; Lennox treats a signal below 1.5 microamps as low. Measuring it needs a meter that reads microamps, so this is a technician check.
Rheem also notes that failed flame sensing can come from wrong polarity, a poor equipment ground or incorrect gas pressure, so a new rod does not always fix it.

Replacing it

Switch off power at the furnace switch and breaker, and shut the gas as your manual directs, before any panel is opened.
Note the rod's position and angle before removing it and fit the new rod the same way.
Restore power and watch a full heating cycle. If the code returns, stop and call a technician: the cause is probably not the rod.

Safety firstGas, venting, high-voltage and control-board work belongs to a licensed HVAC technician. This page is here so you can recognise the part and read a quote, not as a how-to for working on a gas appliance.

Capacitors can hold a charge after the power is off. Safety terms.

Where to buy

These are Amazon links. Product page links go to one specific listing whose title we read on Amazon; we do not say it fits your model. Search links open search results, which change. Always confirm your model number and the part number in your manual.