Gas Furnace Ignition Sensor: Function Troubleshooting and Replacement

The gas furnace ignition sensor plays a critical role in home heating by confirming flame presence and enabling safe operation; this article explains how sensors work, common problems, testing methods, cleaning tips, replacement guidance, and safety considerations to help homeowners and technicians diagnose and resolve issues efficiently.

Component	Common Symptoms	Typical Lifespan	Typical Cost To Replace
Flame Sensor	Short cycling, burners igniting then shutting off	5–15 Years	$50–$150
Hot Surface Ignitor (HSI)	No ignition, furnace fails to start, visible cracks	3–10 Years	$60–$250
Flame Rod (Older Systems)	Intermittent flame detection, error codes	5–15 Years	$40–$120

What A Gas Furnace Ignition Sensor Does

The ignition sensor detects whether the gas burner has successfully ignited and sends that information to the furnace control board.

If the sensor does not detect flame, the control board shuts off the gas valve to prevent unburned fuel buildup, ensuring safe operation.

Different systems use different sensing technologies: flame sensors (metal rod), flame rods, and flame rectification circuits for direct detection, and hot surface ignitors (HSI) for initiating combustion.

Types Of Ignition And Flame Detection Components

Flame Sensor Rod

A flame sensor rod is a small metal strip placed directly in the flame path that uses flame rectification to sense current flow through the flame.

It is common in modern furnaces and is sensitive to dirt and oxidation, which can prevent proper detection.

Hot Surface Ignitor (HSI)

The hot surface ignitor is a ceramic or silicon carbide element that heats until it glows and then ignites the gas; it is not a flame sensor but part of the ignition system.

HSIs fail from thermal stress, cracking, or degradation; while they initiate ignition, separate flame sensors confirm flame presence.

Standing Pilot And Flame Rods

Older furnaces may use a standing pilot with a thermocouple or thermopile to detect flame, using heat-generated voltage instead of rectification.

These components are robust but can degrade from soot, corrosion, or mechanical wear over time.

Common Symptoms Of A Faulty Ignition Sensor

• Short Cycling: Burner lights then shuts off repeatedly.
• Failure To Ignite: Furnace attempts to start but no flame appears.
• Error Codes: Control board diagnostic LEDs or thermostat messages indicating flame failure.
• Long Start Delays: Multiple ignition attempts before flame establishes.
• Visible Damage: Cracks in an HSI or corrosion on a flame sensor rod.

Basic Diagnostic Steps For Homeowners

Before calling a technician, a homeowner can perform safe, non-invasive checks to identify obvious issues.

Check Power And Settings

Confirm the furnace has power, the thermostat is set to heat, and the gas supply is on; reset any tripped switches or breakers.

Inspect Error Codes And LED Patterns

Locate the furnace control board LED or status indicator and compare blink patterns to the service manual to get a diagnostic starting point.

Visual Inspection

With the furnace door closed and power off for safety, look for loose wires, obvious damage to the ignitor, or heavy soot on the flame sensor.

How To Clean A Flame Sensor Safely

Dirty flame sensors are a top cause of false flame-failure shutdowns; cleaning often restores proper function.

1. Turn off power and gas to the furnace.
2. Remove the furnace access panel and locate the flame sensor rod near the burner.
3. Loosen the mounting screw and gently pull the sensor out.
4. Use a fine abrasive pad, emery cloth, or 220-grit sandpaper to remove oxidation; avoid aggressive scraping.
5. Wipe with a clean cloth, reinstall, and restore power and gas.

Do not use solvents that leave residue; only light abrasion and a clean cloth are recommended.

Testing A Flame Sensor With A Multimeter

A multimeter can provide quick electrical verification of sensor continuity or expected voltages for some systems; follow safety precautions.

1. Turn off power to the furnace and remove the sensor.
2. Set the multimeter to the resistance (ohms) range for continuity checks; most flame sensors show very low resistance when functioning.
3. For flame rectification testing, reassemble and measure the microamp rectified current with the furnace running—this is advanced and typically for technicians.

If measurements are out of expected ranges or absent, the sensor likely needs replacement.

When To Replace The Ignitor Versus The Sensor

Replacement decisions depend on the failed component: an HSI that is cracked or non-glowing requires replacement.

A flame sensor that remains dirty despite cleaning, shows heavy corrosion, or fails electrical tests should be replaced.

Replacing the correct component is essential—replacing only the ignitor when the sensor is the issue will not fix flame detection problems.

Step-By-Step Replacement Guide For Flame Sensors

Replacing a flame sensor is often straightforward and suitable for experienced DIYers with basic tools.

1. Turn off power and gas at the main switch and gas valve.
2. Remove the access panel and locate the flame sensor near the burners.
3. Disconnect the sensor wire, remove the mounting screw, and withdraw the sensor.
4. Install the OEM-compatible replacement, tighten the mounting screw snugly, and reconnect the wire.
5. Replace the access panel, restore power and gas, and observe startup; the furnace should light and run normally.

Always use manufacturer-recommended replacement parts and follow safety instructions.

Cost Factors And Typical Prices

Replacement cost depends on part type, furnace model, labor rates, and location; flame sensors are inexpensive while HSIs and labor can increase total cost.

Typical parts cost: flame sensor $20–$100; HSI $60–$250. Technician labor adds $100–$300 depending on complexity and service call fees.

When To Call A Professional

Call a licensed HVAC technician if the homeowner is uncomfortable with electrical work, if ignition problems persist after cleaning, or if there are gas leaks, scorch odors, or repeated safety lockouts.

Technicians can perform safe live-system tests, measure rectification currents, check control board behavior, and verify gas pressure and valve operation.

Safety Considerations And Best Practices

Always disconnect electrical power and shut off gas before accessing internal furnace components; improper handling can lead to electric shock, gas leaks, or fire.

Use only replacement parts specified by the furnace manufacturer and maintain adequate clearance around the unit for ventilation and service access.

Maintenance Tips To Extend Sensor Life

• Schedule annual professional furnace tune-ups to inspect and test sensors and ignition components.
• Replace air filters regularly to reduce dust that can coat the flame sensor.
• Keep the furnace area clean and free of debris that could enter the burner compartment.
• Address intermittent cycling or soot buildup promptly to prevent sensor damage.

Common Misconceptions

One common myth is that replacing the ignitor will fix all ignition problems; however, many issues stem from flame sensors or control boards.

Another misconception is that sensors are maintenance-free; routine checks and cleaning can prevent false shutdowns and improve reliability.

What To Expect During Professional Service

A technician will run diagnostic cycles, test flame current or thermocouple voltage, inspect ignition timing, measure gas pressure, and replace faulty sensors or ignitors with OEM parts.

The service report should document test results, parts replaced, and recommendations for future maintenance to help homeowners track component health.

Replacement Part Selection And Compatibility

Selecting the correct replacement requires matching part numbers, mounting geometry, and electrical connectors to the furnace model; universal parts sometimes fit but can cause fitting or performance issues.

Consult the furnace nameplate, model manual, or manufacturer support to confirm compatibility before ordering parts.

Resources And Further Reading

• Consumer Reports: HVAC Buying & Maintenance Guides
• ASHRAE: Standards And Best Practices
• U.S. Consumer Product Safety Commission: Home Heating Safety

Quick Troubleshooting Checklist

Step	Action
1	Check power, thermostat settings, and gas supply
2	Inspect control board LED error codes and the owner’s manual
3	Visually inspect ignitor and sensor for cracks or heavy soot
4	Clean flame sensor gently if dirty
5	Test with multimeter or call a technician for live testing

For homeowners in the U.S., addressing ignition sensor issues promptly improves safety, reduces heating downtime, and can prevent more costly repairs in cold months when reliable heating is essential.