How to Replace a Furnace Ignitor & Flame Sensor
Complete DIY guide covering hot surface ignitors and flame sensors. Learn diagnosis, testing, and replacement. Includes visual inspection, multimeter testing, and step-by-step procedures. Save $200–$400 on professional repairs.
Important Safety Notice & Disclaimer
Working with gas furnaces carries inherent risk of gas leaks, explosion, carbon monoxide poisoning, burns, and death. The ignitor and flame sensor are located near the burner assembly where gas is present. Gas furnaces run on 120V control circuits but operate with natural gas or propane — any gas leak or improper flame detection is extremely dangerous. Never bypass safety interlocks. Always turn off the gas supply at the shutoff valve AND disconnect power at the breaker before beginning any work. Verify power is off with a multimeter. If you smell gas at any point, immediately shut off the system, leave the home, and call a licensed technician.
This guide is provided for educational purposes only. Open To Public HVAC Parts makes no warranties regarding the accuracy or completeness of this information. By following this guide, you acknowledge that you do so at your own risk. Open To Public HVAC Parts, its owners, and employees are not liable for any injury, damage, loss, or death resulting from the use of this information. If you are not comfortable working with gas appliances or electrical components, hire a licensed HVAC technician.
What Are Ignitors & Flame Sensors?
In a modern gas furnace, the ignitor and flame sensor work together to safely light and maintain the burner flame. Understanding how they work and where they sit in the burner assembly is essential for diagnosing problems and replacing them correctly.
Hot Surface Ignitors (HSI)
A hot surface ignitor is a thin ceramic heating element that glows red-hot when voltage is applied to it. Just before the gas valve opens, the control board energizes the ignitor. Within 3–5 seconds, the element reaches approximately 2,000°F and becomes hot enough to ignite the incoming gas. Once the gas flame is established and detected by the flame sensor, the ignitor is de-energized and shuts off. The ignitor looks like a small ceramic rod or pencil-shaped element, usually white or tan in color, and is mounted near the burners in the burner assembly.
Flame Sensors
A flame sensor is a thin metal rod (typically stainless steel or iron) that sits in the path of the flame. When exposed to an open flame, the sensor detects it through flame rectification — the sensor conducts current in the presence of a flame, sending a tiny millivolt signal back to the control board. This signal tells the board the flame is present and safe to continue operation. If the flame goes out or the sensor loses sight of the flame, the board immediately shuts off the gas valve and enters a lockout sequence to prevent dangerous buildup of unignited gas.
Location & Appearance
Both components are mounted in or very near the burner assembly, below the heat exchanger. The ignitor is typically positioned directly in front of the burner ports or gas jets. The flame sensor is positioned nearby, also with a direct line of sight to the flames. While they sit close together, they do very different jobs: the ignitor creates the heat to light the gas, while the flame sensor monitors that a safe, sustained flame exists. Do not confuse them — replacing the wrong part will not fix your problem.
Signs Your Ignitor or Flame Sensor Is Bad
Signs of a Failing Ignitor
- Furnace clicks or tries to start but no ignition occurs: You hear the inducer motor and gas valve click, but the flame never lights. The ignitor may be dead or too weak to reach ignition temperature.
- No visible glow through the sight glass: If your furnace has a burner sight glass, you should see the ignitor glow red during the ignition sequence. No glow = bad ignitor.
- Cracked or broken ignitor element: Visually inspect the ignitor for visible cracks, breaks, or a dull, whitish coating on the element (sign of age/failure).
- Furnace lockout code: The control board LED blinks a fault code such as “flame failure,” “ignition failure,” or “prove failure.” Check your furnace manual for the code sequence.
- Repeated ignition attempts then shutdown: The furnace keeps trying to light, failing each time, then locks out after 3–5 attempts.
Signs of a Failing Flame Sensor
- Furnace lights then shuts off within 3–7 seconds: The ignitor glows, gas lights, but immediately after the ignitor shuts off, the gas valve closes and the flame dies. Classic flame sensor symptom.
- Repeated ignition attempts with short flame duration: Each time the system tries to start, the flame lights briefly then cuts off. This cycle repeats until lockout.
- Flame sensor error codes: The control board blinks codes indicating “flame loss,” “sensor failure,” or “proving failure.”
- Flame sensor dirty or corroded: Visual inspection shows a black, sooty, or heavily oxidized flame sensor surface. A dirty sensor cannot detect flame properly.
How to Diagnose Ignitor & Flame Sensor Problems
Check the Fault Code on the Control Board
Before you do anything else, check the blinking LED on the control board. Most furnaces display diagnostic codes. Count the blinks: short blinks separated by longer pauses indicate different faults. Common codes: 3 blinks = flame loss, 4 blinks = ignition failure. Write down what you see. Consult your furnace manual or search online for the code meaning. This often tells you immediately whether to suspect the ignitor or flame sensor.
Visual Inspection of the Ignitor
With power OFF: Remove the burner access panel carefully. Look at the ignitor element. It should appear white or tan, smooth, and intact. Look for cracks, breaks, or a dull, chalky coating. If it’s obviously cracked or dark/discolored, it’s bad and needs replacement. If it looks fine visually, proceed to electrical testing.
Multimeter Resistance Test on Ignitor
With power OFF: Set your multimeter to resistance (ohms). Disconnect the wire connector from the ignitor (if possible). Touch one probe to each ignitor terminal. A good hot surface ignitor reads between 40 and 200 ohms. If you see “OL” (open line / infinite resistance), the element is broken and the ignitor is bad. If the resistance is extremely low or zero, the ignitor may have an internal short. Replace it if out of range.
Flame Sensor Visual Inspection & Cleaning
With power OFF: Locate the flame sensor (thin metal rod near the burners). Inspect it for heavy soot, dirt, or corrosion. A black, crusty, or heavily oxidized surface indicates contamination. Many flame sensor problems are simply dirt blocking the sensor’s view of the flame. Remove the single mounting screw holding the sensor. Pull it out gently. Use a fine emery cloth or fine steel wool to gently scrub the sensor surface until it’s shiny. Do not scratch or pit the surface. Reinstall and test. If cleaning doesn’t solve the problem, the sensor may be faulty and need replacement.
Flame Sensor Microamp Test (Advanced)
This requires a specialized meter and HVAC knowledge. Set your multimeter to microamps (µA) if available. With the system running and the flame lit, measure the current flowing through the flame sensor circuit. A good flame sensor typically reads 1–10 microamps. Below 1 µA suggests a weak or failing sensor. This test requires live voltage, so only attempt if you’re comfortable working with energized equipment.
Ignitor & Flame Sensor Location in Gas Furnace Burner Assembly
Tools & Parts You’ll Need
Tools
- Multimeter (for resistance and voltage testing)
- Phillips and flathead screwdrivers
- 1/4″ and 5/16″ nut drivers or socket wrench
- Fine emery cloth or fine steel wool (for cleaning flame sensor)
- Needle-nose pliers (for wire connectors)
- Work gloves and safety glasses
- Phone/camera (to photograph wiring before disconnecting)
- Flashlight or headlamp (burner area is dark)
Parts
- Replacement hot surface ignitor (if needed — must match your furnace model)
- Replacement flame sensor (if needed — universal types available)
- Wire connectors or spade terminals (if original connectors are damaged)
Step-by-Step: Replacing the Ignitor
Turn Off All Power & Gas
Flip the power switch on the furnace to OFF. Turn off the breaker at the electrical panel. Close the manual gas shutoff valve (usually a red or black lever on the gas line). Use a multimeter to verify there’s no voltage at the control board. Do not proceed until you are absolutely certain power and gas are off.
Remove the Burner Access Panel
Carefully remove the lower burner access panel (usually 1–2 screws or a latch). Set it aside. You should now have clear access to the burner assembly, ignitor, and flame sensor. Use a flashlight to see into the burner area clearly.
Photograph the Wiring
Take clear photos of the ignitor wire connector and how it’s routed. Note any labels on the wires. This helps you reconnect the new ignitor correctly. Also photograph the flame sensor location and wiring. These photos are your safety net.
Disconnect the Ignitor Wire Connector
Locate the wire connector on the ignitor (usually spade connectors or a small plug). Gently pull the connector apart. If the connector is old and brittle, support the ignitor wires to avoid stressing them. Set the connector aside — you may reuse it or may need a new one depending on condition.
Remove the Ignitor Mounting Screw
The ignitor is held in place by one or two small screws (usually 1/4″ or 5/16″). Use the appropriate nut driver or screwdriver to remove them. Support the ignitor element gently as you remove the last screw so it doesn’t fall or break. Carefully slide the old ignitor out of its mounting bracket.
Install the New Ignitor
Carefully insert the new ignitor into the mounting bracket, aligning the screw holes. CRITICAL: Do not touch the ceramic ignitor element with bare hands. Skin oils cause hot spots on the element during operation, leading to premature failure and cracking. If you must handle it, wear clean cotton gloves. Reinstall the mounting screws snugly (don’t over-tighten or you may crack the ceramic). The ignitor should sit at the correct angle and position relative to the gas burner ports — consult your furnace manual or reference photos if in doubt.
Reconnect the Wire Connector
Push the wire connector back onto the ignitor terminals until it seats firmly. If the connector is damaged, cracked, or worn, replace it with a new one. Ensure the connection is tight and clean. Loose connections cause intermittent ignition failures.
Reinstall the Burner Access Panel & Test
Replace the burner access panel and secure it. Restore power at the breaker. Open the gas shutoff valve. Turn the furnace power switch back on. Set your thermostat to call for heat. Watch and listen as the furnace goes through the startup cycle. You should see the ignitor glow red-hot, hear a click as the gas valve opens, and see flames light across the burners. If all looks good, the replacement is successful. Run the system for 5–10 minutes to ensure it operates normally.
Step-by-Step: Cleaning or Replacing the Flame Sensor
Turn Off All Power & Gas
Flip the furnace power switch to OFF. Turn off the breaker at the electrical panel. Close the manual gas shutoff valve. Use a multimeter to confirm no voltage is present. Wait 15 minutes to ensure the burner area cools.
Remove the Burner Access Panel
Remove the lower burner access panel (1–2 screws or latch). You now have access to the burner assembly where the flame sensor sits. Use a flashlight to locate the flame sensor — it’s a thin metal rod, usually stainless steel or iron, positioned near the burners with a line of sight to the flames.
Remove the Flame Sensor Mounting Screw
The flame sensor is held in place by a single mounting screw (usually 5/16″ or 1/4″). Use the appropriate nut driver or screwdriver to remove it. Support the flame sensor as you remove the screw so it doesn’t fall into the burner area. Gently pull the sensor out of its bracket. Note the orientation — the sensor must be reinstalled pointing toward the flames.
Clean the Flame Sensor (If Contaminated)
Inspect the sensor surface. If it’s coated with black soot, dirt, or corrosion, cleaning may restore function. Use a soft, fine emery cloth or very fine steel wool. Gently scrub the surface until it’s shiny and clean. Do not scratch, pit, or gouge the surface — you want a smooth finish. Wipe away any dust with a clean, dry cloth. Reinstall the sensor in its bracket, ensuring it points toward the burners and seats fully.
Test After Cleaning
Reinstall the burner access panel. Restore power and gas. Set the thermostat to call for heat. Run a full heating cycle. If the furnace now lights, stays lit, and runs normally, cleaning solved the problem. If it still fails or lights then shuts off within seconds, the sensor is faulty and needs replacement (proceed to step 6).
Replace the Flame Sensor (If Cleaning Didn’t Work)
If cleaning didn’t restore operation, the sensor element is worn out. Turn off power and gas again (same as step 1). Remove the burner panel and the old flame sensor. Obtain a replacement flame sensor — universal types are available and work in most furnaces. Install the new sensor in the same bracket and orientation as the old one. The mounting screw should be snug but not over-tightened. Ensure the sensor has a clear line of sight to where the flames will be.
Reinstall & Test
Replace the burner access panel. Restore power and gas. Set the thermostat to call for heat. Observe a full startup cycle. The furnace should light, the ignitor should glow, the gas valve should open, flames should ignite, and the system should stay on. If it lights and runs for a normal heating cycle without shutting off, the replacement is successful.
Cost Comparison: DIY vs. Professional Service
| Component | DIY Part Cost | Professional Service Cost | Savings (DIY) |
|---|---|---|---|
| Hot Surface Ignitor | $15 – $40 | $150 – $350 | $110 – $310 |
| Flame Sensor | $15 – $30 | $150 – $300 | $120 – $270 |
| Both Components | $30 – $70 | $300 – $650 | $230 – $620 |
Note: Professional costs include labor, diagnostic fees, and service calls. DIY costs reflect parts only. Savings vary by region and HVAC company rates.
Video Guides & Demonstrations
How to Test Hot Surface Ignitors — 2 Ways
Learn two practical methods to test an ignitor with a multimeter and by visual observation. This video covers resistance testing and voltage testing to determine if your ignitor is good or bad.
Flame Sensor vs Ignitor — How To Identify Furnace Parts
Clear explanation of the difference between flame sensors and hot surface ignitors, where they’re located, and what they do. Essential for understanding which part to replace.
How Your Gas Furnace Works — Complete Breakdown for DIY Repair
A comprehensive walkthrough of the gas furnace operation sequence, including the role of the ignitor, flame sensor, gas valve, and control board. Understanding how it all works together makes diagnosis easier.
Need an Ignitor or Flame Sensor?
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