Furnace Flame Sensor Symptoms: Signs, Causes, and Fixes

The flame sensor is a small but critical component that tells a furnace the burner is lit. Recognizing furnace flame sensor symptoms early can prevent repeated system lockouts, energy waste, and safety concerns. This article outlines common symptoms, diagnostic steps, DIY fixes, costs, and when to call a professional to restore reliable heating.

Symptom	What It Indicates	Action
Frequent Furnace Short Cycling	Sensor Not Detecting Flame	Inspect/Clean Or Replace
Furnace Ignites Then Shuts Off	Intermittent Flame Signal	Check Sensor Alignment/Clean
Hot Surface Igniter Tries Repeatedly	Continuous Lockouts	Test Sensor Voltage/Replace
No Heat But Blower Runs	Burner Not Confirmed By Sensor	Diagnose Flame Sensor Circuit

What Is A Furnace Flame Sensor And Why It Matters

The flame sensor is a thin metal rod positioned in the burner assembly that senses whether the gas burner flame is present. If the sensor does not register a flame within seconds, the furnace control board shuts the gas valve to prevent unburned gas buildup. Reliable flame sensing is essential for safe furnace operation and efficient heating performance.

Common Furnace Flame Sensor Symptoms

Frequent Short Cycling Or Lockouts

If the furnace starts, lights the burners, then shuts off after a few seconds and repeats, this indicates the control board is not receiving a steady flame signal from the flame sensor. Repeated lockouts are one of the most common symptoms.

Ignites Then Immediately Shuts Off

When the burner lights but goes out shortly after ignition, the flame sensor may be intermittently failing to detect the flame. Dirt, rust, or misalignment can cause this pattern.

No Heat While Blower Runs

The blower might continue operating because the thermostat calls for heat, but the burners never stay lit due to the flame sensor failing to confirm combustion. This results in warm air not being produced.

Long Ignition Sequence Or Repeated Ignition Attempts

If the furnace repeatedly attempts to ignite or takes much longer than normal to confirm combustion, the control board receives inconsistent flame signals. That is a clear furnace flame sensor symptom.

Yellow Or Weak Burner Flame

A healthy gas flame should be mostly blue. A weak, flickering, or yellow flame can prevent proper sensing and may point to burner or sensor issues, including sensor contamination or gas pressure problems.

How A Flame Sensor Works

The flame sensor generates a small electrical current when exposed to a flame due to the flame rectification process. The furnace control board measures this microamp signal to confirm combustion. If the signal falls below a threshold, the board shuts off the gas valve to prevent unsafe conditions. Accurate electrical sensing is therefore critical to furnace safety.

Common Causes Behind Flame Sensor Symptoms

Contamination And Carbon Buildup

Carbon deposits and oxidation on the sensor tip are the most common causes of flame sensing failure. Even thin layers of residue can interrupt signal flow and trigger symptoms.

Sensor Misalignment Or Loose Mounting

If the sensor is not correctly positioned in the burner flame or its mounting screws are loose, it may not be exposed to consistent flame and the control board will not register the flame signal.

Electrical Issues

Wiring faults, corroded connectors, or a failing control board input can mimic flame sensor failure. Testing the sensor circuit is necessary to isolate the problem.

Age And Mechanical Wear

Older sensors can degrade over time, with metal thinning or connections failing. Age-related wear may require replacement rather than cleaning.

Bad Burner Performance

Poor combustion, dirty burners, or improper gas pressure can produce unstable flames that the sensor cannot detect reliably, producing similar symptoms.

How To Diagnose Furnace Flame Sensor Symptoms Safely

Diagnosing should follow safety protocols: turn off power and gas before inspecting components. Use a multimeter and, if available, a microamp meter to measure the flame signal. Never work on gas appliances without proper precautions or certification for complex repairs.

Visual Inspection

Inspect the flame sensor for carbon buildup, discoloration, or physical damage. Check that it is securely mounted and positioned within the flame path.

Cleaning The Sensor

Lightly sanding the tip with fine-grit emery cloth or using a scouring pad can remove deposits. After cleaning, wipe with a dry cloth to remove residue and test operation.

Electrical Testing

Using a multimeter, confirm there is proper continuity and no open circuit in the sensor wiring. With the furnace running, measure the flame sensing current (typically 2–10 microamps). Values below the manufacturer’s threshold indicate sensor or circuit problems.

Swap Or Replace For Confirmation

If cleaning and wiring checks do not resolve symptoms, testing with a known-good sensor (if available) or replacing the sensor can confirm the diagnosis. Flame sensors are usually inexpensive and often restore normal operation when faulty.

DIY Fixes And Step-By-Step Cleaning

Many furnace flame sensor symptoms are resolved by cleaning. Homeowners can perform basic maintenance if comfortable and careful. If not comfortable, contact a licensed HVAC technician.

1. Turn off thermostat, power to the furnace at the breaker, and shut off the gas supply.
2. Open the furnace access panel and locate the flame sensor near the burners.
3. Remove the mounting screw(s) and gently pull out the sensor.
4. Use fine-grit sandpaper or a non-abrasive pad to gently clean the metal rod; avoid deep scraping.
5. Wipe the sensor clean with a dry cloth and reinsert, ensuring proper position and tight mounting.
6. Restore power and gas, then run the furnace to verify normal ignition and no lockouts.

Important safety note: Do not use harsh chemicals or immerse the sensor. If problems persist after cleaning, stop and call a professional.

When To Call A Professional

Contact an HVAC technician if the furnace flame sensor symptoms continue after cleaning, if electrical testing is needed, or if there are signs of gas leaks, persistent yellow flames, or control board errors. Professionals can safely perform in-depth diagnostics, control board testing, and gas pressure checks.

Preventive Maintenance To Avoid Future Symptoms

Regular furnace maintenance reduces flame sensor problems. Schedule annual tune-ups before the heating season to clean burners, check the flame sensor, tighten connections, and test safety controls.

• Change air filters regularly to reduce dust draw into the burner area.
• Keep the furnace area clean to minimize debris and carbon buildup.
• Address combustion issues promptly—yellow flames often indicate combustion problems that affect sensing.

Parts, Replacement Costs, And Time Estimates

Flame sensors are typically inexpensive parts. Homeowners can expect parts costs between $15 and $60 depending on model and furnace brand. Professional service including diagnosis and replacement usually ranges from $100 to $300, depending on travel, labor rates, and any additional repairs.

Item	Typical Cost	Time
Flame Sensor Part	$15–$60	10–30 Minutes To Replace
Technician Service Call	$100–$300	30–90 Minutes
Full Tune-Up	$100–$250	1–2 Hours

Troubleshooting Checklist For Technicians

Technicians diagnosing furnace flame sensor symptoms should follow a systematic checklist to avoid missed causes. Essential steps include burner inspection, sensor cleaning, microamp measurement, wiring inspection, and control board verification.

1. Verify symptom reproducibility and control board error codes.
2. Inspect flame pattern and burner condition.
3. Clean or replace the flame sensor and recheck signal amplitude.
4. Measure flame sensing current; compare to specifications.
5. Check wiring, terminal connections, and ground integrity.
6. Evaluate control board inputs/outputs and replace if necessary.

Frequently Asked Questions About Furnace Flame Sensor Symptoms

Can Cleaning The Flame Sensor Fix All Symptoms?

Cleaning resolves many issues caused by carbon buildup, but not problems stemming from wiring faults, control board failures, or poor combustion. If symptoms persist, further electrical and combustion diagnostics are needed.

Is A Flame Sensor The Same As A Flame Rectifier?

The flame sensor uses the principle of flame rectification to detect flame presence. Terminology varies, but both describe the component that confirms combustion to the control board.

How Often Should The Flame Sensor Be Serviced?

During annual furnace maintenance, the flame sensor should be inspected and cleaned. High-use systems or dusty environments may need more frequent checks.

Can A Dirty Flame Sensor Be A Safety Hazard?

Yes. A malfunctioning flame sensor can cause repetitive gas valve cycling and potential accumulation of unburned gas if the system were to fail to detect flame, so addressing symptoms promptly is important for safety.

Key Takeaways For Addressing Furnace Flame Sensor Symptoms

Early recognition of furnace flame sensor symptoms—such as repeated lockouts, short cycling, and ignition failures—can save time and avoid hazards. Basic cleaning and inspection often restore normal operation, but persistent issues require professional electrical or combustion diagnostics. Regular maintenance reduces recurrence and keeps the heating system safe and efficient.