Flame Sensor Not Working? How to Clean and Replace It
If your furnace lights up and then shuts off after a few seconds, a dirty or failed flame sensor is the most likely cause. The flame sensor is a small metal rod that sits in the burner flame and tells the control board that gas is actually burning. When it can’t detect a flame — usually because of carbon buildup on the rod — the control board shuts off the gas valve as a safety measure. It’s one of the most common furnace repairs, and one of the easiest to do yourself.
How the Flame Sensor Works
The flame sensor works by detecting a tiny electrical current — measured in microamps — that flows through the flame. A clean sensor in a good flame produces around 2 to 6 microamps. If that reading drops below about 1 microamp (because the sensor is dirty or the flame is weak), the control board interprets it as “no flame” and shuts down the gas. This is a critical safety feature — without it, unburned gas could fill your house.
Signs of a Dirty Flame Sensor
The classic symptom is a furnace that lights, runs for 3 to 8 seconds, then shuts off. The ignitor glows, the gas valve opens, the burners light — and then everything shuts down. The furnace might retry this sequence two or three times before locking out entirely, requiring a power reset. If this describes your furnace, the flame sensor should be your first check.
How to Clean It
Step 1: Turn off power to the furnace at the switch or breaker. Turn off the gas supply at the valve near the furnace.
Step 2: Locate the flame sensor — it’s a thin metal rod (usually bent at a 90-degree angle) mounted to the burner assembly with a single 1/4-inch hex screw. It has one wire connected to it. On most furnaces, it’s near the end of the burner tube opposite the ignitor.
Step 3: Remove the single mounting screw and gently pull the sensor out. Disconnect the wire if needed.
Step 4: Clean the metal rod with fine steel wool, an emery cloth, or a dollar bill. Rub gently until the rod is shiny. You’re removing the oxidation and carbon buildup that insulates the rod and prevents it from sensing the flame current.
Step 5: Reinstall, reconnect the wire, restore gas and power, and test. In most cases, a good cleaning fixes the problem immediately.
When to Replace Instead of Clean
If the sensor is cracked, the porcelain insulator is damaged, or cleaning doesn’t fix the issue, it’s time for a new one. Flame sensors are inexpensive — typically $15 to $30 at Open To Public HVAC Parts. Bring your old sensor in or give us the furnace model number and we’ll match the replacement. Some sensors are universal, while others are specific to the manufacturer.
Other Causes of the Same Symptom
If a clean or new sensor still doesn’t fix the light-then-shutdown cycle, the problem could be a weak flame (dirty burners or low gas pressure), a failing control board that isn’t reading the sensor signal properly, or a cracked heat exchanger allowing draft to blow out the flame. The sensor is the right first step because it’s the cheapest and most common fix.
Bring in your flame sensor and we’ll inspect it at the counter. If it’s too far gone, we’ll get you matched with a replacement in minutes.
Open To Public HVAC Parts — 10226 Plano Rd, Suite 104, Dallas, TX 75238. Call or text (214) 340-9421. Open Monday–Friday 10–7, Saturday 10–3.
