Furnace Flame Sensor: Function, Troubleshooting, and Replacement Guide

The furnace flame sensor is a small but critical safety device that detects whether a gas furnace’s burner is lit. When it fails, the furnace may short-cycle, not heat, or lock out for safety. This article explains how the flame sensor on furnace systems works, common problems, step-by-step testing, cleaning and replacement guidance, and tips to prevent future issues.

Symptom	Likely Cause	Quick Fix
Furnace lights then shuts off	Dirty Or Faulty Flame Sensor	Clean Or Test Sensor
Furnace Won’t Light	Sensor Failure Or Wiring Issue	Check Connections, Replace If Needed
Frequent Ignition Attempts	Intermittent Sensor Signal	Inspect, Clean, Replace

How The Flame Sensor On Furnace Systems Works

The flame sensor on a furnace is a metal rod positioned in the burner flame that detects the presence of flame via a small electrical current caused by flame rectification. When the control board calls for heat, the igniter lights the gas, and the sensor verifies combustion within seconds. If the control board doesn’t see the expected signal, it shuts the gas off to prevent unburned gas buildup.

Key Function: Provide a safety confirmation that burners are lit, preventing dangerous gas flow when flame is absent.

Common Types And Locations Of Flame Sensors

Most residential furnaces use a single straight metal probe flame sensor mounted near the burner assembly. Alternative systems may use rod, probe, or flame-sensing electrodes integrated with the igniter or pilot assembly. The sensor is typically located in front of the burners inside the combustion chamber, accessible by removing the furnace access panel.

Signs Of A Failing Or Dirty Flame Sensor

Recognizing symptoms early reduces service costs and prevents repeated furnace lockouts. Common signs include:

• Furnace Starts Then Shuts Off After Few Seconds
• System Ignites Several Times Before Staying On
• Frequent Error Codes Or Lockouts On Control Board
• Failure To Heat Despite Ignition Attempts

Note: These symptoms also overlap with other ignition problems, so proper testing is essential before replacing parts.

Safety Precautions Before Servicing A Flame Sensor

Always prioritize safety. Turn off power to the furnace at the switch or breaker and shut off the gas supply when performing inspections or maintenance. Wear gloves and eye protection. If uncomfortable with electrical diagnostics or gas components, contact a licensed HVAC technician.

Tools And Materials Needed

Essential items for diagnosing and servicing a furnace flame sensor include:

• Multimeter (for continuity and microamp measurement)
• Fine-grit sandpaper (400–600 grit) or emery cloth
• Soft cloth or brass brush (nonferrous)
• Screwdrivers (typically 1/4″ nut driver or Philips)
• Replacement flame sensor compatible with furnace model

Step-By-Step: Testing A Flame Sensor On Furnace

Testing distinguishes between a dirty sensor and a failed sensor. Follow these steps after turning electrical power off, then on when instructed for tests that require live circuits.

Visual Inspection

Inspect the sensor rod for heavy soot, corrosion, or physical damage. Check wiring and connector for loose or burnt contacts. A visibly damaged rod requires replacement rather than cleaning.

Voltage/Current Test With Multimeter

Set the multimeter to the DC microamp range if available. Restore power and call for heat so the burners ignite. Carefully back-probe the flame-sensing circuit per furnace wiring diagram and measure the flame current. Typical flame currents range from 1 to 10 microamps, depending on furnace design. A stable measurable current indicates a working sensor; near-zero or fluctuating reading suggests a problem.

Control Board Signal Check

Some systems allow measuring between the sensor terminal and ground to see the rectified signal. Consult the furnace manual for exact test points. If unsure, leave testing to a technician to avoid board damage.

Cleaning The Flame Sensor: When And How

Cleaning is a low-cost first step when the sensor shows signs of contamination but remains physically intact. It often resolves intermittent failures caused by carbon build-up or oxidation.

Proper Cleaning Method

Turn off power and gas. Remove the sensor by unscrewing its mounting screw and gently withdraw it. Lightly abrade the metal probe with fine-grit sandpaper or emery cloth to remove soot and film. Wipe with a clean cloth to remove debris. Do not use harsh chemicals, flame, or heavy sanding that changes probe shape. Reinstall, restore power, and test furnace operation.

Important: Over-cleaning can thin the rod or alter its geometry, causing poor sensing — replace if the probe looks worn.

When To Replace The Flame Sensor On Furnace

Replace the flame sensor if any of the following applies:

• Multimeter shows negligible flame current after proper cleaning
• Probe is pitted, corroded, or physically damaged
• Wiring or connector is melted or corroded beyond repair
• Sensor repeatedly fails despite cleaning and secure wiring

Replacement sensors are inexpensive parts. Choose one specified for the furnace make and model or a direct-equivalent universal sensor with the same probe length and mounting style.

Replacement Procedure

Replacement is straightforward for most homeowners with basic hand tools. Turn off power and gas. Remove access panels to reach the sensor. Disconnect the wire harness and remove the mounting screw. Install the new sensor in the same orientation, secure the screw snugly, and reconnect wiring. Restore power and gas, then observe several heat cycles to confirm normal operation.

Common Mistakes To Avoid

Avoid these pitfalls to ensure successful repairs:

• Cleaning With Harsh Chemicals Or A Wire Brush That Damages The Rod
• Over-Tightening The Mounting Screw Which Can Crack Ceramic Insulation
• Failing To Check Wiring And Connectors—Bad Wiring Can Mimic Sensor Failure
• Replacing The Sensor Without Verifying Other Ignition Components

Maintenance Tips To Prevent Future Flame Sensor Issues

Regular maintenance reduces flame sensor problems. Recommended practices include:

• Annual Furnace Inspection By A Qualified Technician
• Keeping Return Vents And Filters Clean To Reduce Soot
• Checking Combustion Air Supply And Venting To Prevent Incomplete Combustion
• Cleaning The Flame Sensor During Routine Service If Contaminated

How Furnace Age And Operation Affect The Sensor

Older furnaces may produce more soot or have degraded burners, increasing sensor contamination. Intermittent flame sensing can also indicate burner misalignment or manifold pressure issues. Consider evaluating the entire combustion system if recurring flame sensor failures occur.

When To Call A Professional

Contact an HVAC technician when:

• Electrical measurements are uncertain or unsafe
• There is suspected gas valve or ignition module failure
• Furnace repeatedly locks out after replacing or cleaning the sensor
• Comfort or indoor air quality issues accompany ignition problems

Licensed professionals can perform advanced diagnostics such as combustion analysis, multimeter microamp testing, and control board troubleshooting.

Cost Considerations And Typical Replacement Prices

Replacement flame sensors are generally inexpensive parts, often between $20 and $60 for the component. Labor costs vary; a professional service call with part replacement typically ranges from $100 to $300 depending on region and service complexity. DIY replacement saves labor fees but should only be performed by those comfortable with gas appliance safety.

Troubleshooting Quick Checklist

Use this quick checklist to isolate flame sensor problems:

1. Observe furnace behavior and error codes.
2. Perform visual inspection for soot, corrosion, or damage.
3. Clean the sensor if contaminated and retest.
4. Measure flame current with a multimeter if equipped to do so safely.
5. Check wiring, connectors, and control board terminals.
6. Replace sensor if current remains absent or probe is damaged.

Frequently Asked Questions About Furnace Flame Sensors

How Long Does A Flame Sensor Last?

With proper maintenance, a flame sensor can last many years; however, exposure to combustion byproducts and corrosion can shorten its useful life. Expect multi-year lifespans, but monitor performance annually.

Can A Dirty Flame Sensor Cause Carbon Monoxide Leaks?

A dirty flame sensor itself does not cause carbon monoxide leaks, but underlying combustion problems that lead to soot or improper burning can increase CO risk. Regular professional combustion testing helps ensure safe operation.

Is It Safe To Run A Furnace With A Faulty Flame Sensor?

Modern furnaces will lock out if the flame sensor fails, preventing gas flow when no flame is detected. Do not attempt to bypass or disable safety controls; always repair or replace the sensor promptly.

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If further assistance is needed, consult the furnace’s manual for model-specific instructions or contact a licensed HVAC professional for safe diagnostics and repair.