Furnace Pilot Light Is on but Not Producing Heat

The furnace pilot light being lit but producing no heat can be frustrating and potentially risky. This article explains the most common causes, step-by-step troubleshooting, safety precautions, likely repairs, and when to call a professional. It helps homeowners quickly identify issues and make informed decisions about repair or replacement.

Symptom	Possible Causes	Typical Fix
Pilot Light On, No Heat	Thermostat Issues, Dirty Pilot Orifice, Faulty Gas Valve, Blocked Vent, Failed Limit Switch, Ignition Control Fault	Thermostat Reset/Replace, Clean Orifice, Replace Gas Valve, Unblock Vent, Replace Switch, Call Technician

How A Pilot Light Furnace Works

A pilot light furnace uses a small, continuously burning flame to ignite the main burner when heat is needed. The thermostat signals the furnace to open the gas valve to the burners. The pilot lights the burners and the heat exchanger transfers warmth to the ductwork and living spaces.

Key Components: pilot light, thermocouple or flame sensor, gas valve, thermostat, limit switch, draft/vent system. Any failure in these parts can allow the pilot to remain lit while the furnace fails to produce heat.

Common Reasons The Pilot Light Is On But There’s No Heat

Thermostat Problems

A malfunctioning thermostat can fail to send a call for heat even though the pilot is burning. Settings, wiring faults, low batteries in digital thermostats, or incorrect mode selection often explain the issue.

Faulty Gas Valve Or Control Board

If the gas valve or control board fails, the pilot flame may remain on but the valve to the main burners won’t open. Symptoms include no clicking or gas flow when the thermostat calls for heat.

Dirty Or Clogged Pilot Orifice

A partially clogged pilot orifice can produce a weak flame that isn’t hot enough to reliably ignite the main burners. Soot, dust, and spider webs are common obstructions.

Bad Thermocouple Or Flame Sensor

The thermocouple or flame sensor detects the pilot flame and keeps the gas valve open. A failing or dirty sensor may intermittently prevent burner ignition while still allowing the pilot to burn.

Blocked Venting Or Flue Problems

Blocked flues or blocked intake air can trigger safety switches and prevent the burners from operating. Draft issues can cause the furnace to shut down the main burners while leaving the pilot lit.

Tripped Limit Switch Or Safety Switches

Limit switches protect against overheating and may disable burners if the furnace detects unsafe conditions. A stuck or faulty switch can shut off the main burner circuit while the pilot remains lit.

Ignition Control Module Failure

Some furnaces use an ignition control module to sequence ignition operations. A failed module can prevent the burner from lighting even with the pilot present.

Gas Supply Issues

Partial blockages or supply interruptions between the gas meter and the main burner valve can restrict flow to the burners but still allow enough gas for the pilot light.

Safety First: Precautions Before Troubleshooting

Always prioritize safety. If there is a strong smell of gas, evacuate the home and call the gas company or emergency services from a safe location. Do not attempt repairs that involve gas-line replacement or complex electrical work without proper licensing.

Recommended Safety Steps: turn off power to the furnace before accessing internal parts, shut off the gas supply if instructed in a checklist, ensure adequate ventilation, and wear protective gear when working near burners.

Step-By-Step Troubleshooting Guide

1. Confirm Thermostat Settings And Power

Set the thermostat to heat and raise the setpoint above room temperature. Check batteries, thermometer calibration, and the thermostat mode (Heat vs Off or Cool). Replace batteries and verify the display and wiring connection.

2. Listen For Furnace Operation

When the thermostat calls for heat, listen for the furnace to start: you may hear a relay click, inducer motor run, or gas valve open. No sound can indicate a control or power issue.

3. Inspect The Pilot Flame

Examine the pilot flame: it should be steady and blue with a small inner cone. A yellow, flickering, or weak flame suggests dirt or air/gas pressure problems. Cleaning the pilot assembly can sometimes restore proper ignition.

4. Check The Flame Sensor Or Thermocouple

Turn off power and gas, remove and gently clean the flame sensor or thermocouple with fine emery cloth to remove film and soot. Reinstall and test. A replacement may be necessary if cleaning doesn’t help.

5. Test The Gas Valve And Ignition Sequence

If the thermostat calls for heat and the pilot stays lit but burners don’t ignite, the gas valve or ignition control may be at fault. Visual checks can detect corrosion or burn marks on the control board, but testing often requires a multimeter and professional knowledge.

6. Inspect Ventilation And Draft Systems

Check the flue, vent pipes, and exterior vent caps for obstructions like nests or debris. Ensure combustion air intake vents are clear. Blocked vents can activate safety switches that block burner operation.

7. Look For Error Codes Or Diagnostic Lights

Modern furnaces include diagnostic LEDs or control boards that flash error codes. Consult the furnace manual for interpretation. Codes often point directly to pressure switches, limit switches, or sensor failures.

8. Evaluate Limit Switches And Safety Switches

Examine limit switch locations for visible damage or tripped positions. Resetting may be possible, but recurrent trips indicate underlying problems such as restricted airflow or control failures.

Common DIY Fixes And When They Work

• Thermostat battery replacement or reprogramming usually fixes control-related issues.
• Cleaning the pilot orifice and flame sensor often restores reliable ignition when contamination is the cause.
• Clearing vent blockages and ensuring adequate airflow can resolve safety switch trips and restore burner operation.

Note: Tasks involving gas valves, internal wiring, or component replacement require professional service for safety and code compliance.

When To Call A Professional HVAC Technician

Call a licensed HVAC technician if troubleshooting doesn’t restore heat or if any of these conditions exist: persistent gas smell, visible damage to the gas valve or control board, failed safety switches, repeated pilot outages, or if the homeowner is uncomfortable working with gas and electrical systems.

Professional Diagnostics Include: pressure testing the gas line, checking manifold and pilot pressures, using multimeters to test control circuits, interpreting error codes, and replacing failed components to code-compliant standards.

Estimated Repair Costs And Timeframes

Issue	Typical Cost Range (US)	Typical Time
Thermostat Battery/Reset	$0–$75	15–30 Minutes
Clean/Replace Flame Sensor Or Thermocouple	$75–$200	30–60 Minutes
Replace Gas Valve	$300–$900	1–3 Hours
Control Board Replacement	$200–$700	1–2 Hours
Flue/Vent Cleaning	$100–$400	1–3 Hours

Prices vary by region, furnace model, and whether emergency or same-day service is required. Always obtain multiple quotes for major repairs or replacements.

Preventive Maintenance To Avoid Future Issues

Annual professional furnace tune-ups significantly reduce the risk of pilot-related failures. Maintenance typically includes cleaning burners and orifices, inspecting and cleaning flame sensors, checking gas pressure, testing safety switches, and verifying thermostat operation.

• Change filters regularly to maintain airflow and prevent limit switch trips.
• Keep the furnace area clean and free of debris to prevent blockages and fire hazards.
• Schedule annual inspections before the heating season to catch wear and prevent mid-winter breakdowns.

When Replacement Might Be Better Than Repair

Repair may not make sense for older furnaces nearing end-of-life, frequent breakdowns, or when repair cost approaches the value of a new, high-efficiency furnace. Consider replacement if efficiency is low, repair frequency is high, or safety concerns exist.

Replacement Considerations: higher efficiency models reduce energy bills, new units meet current safety standards, and modern systems often include better diagnostics and reliability.

Frequently Asked Questions

Why Does The Pilot Stay Lit But Burners Won’t Ignite?

Common reasons include a faulty gas valve, failing flame sensor, clogged pilot orifice, control board failure, or safety switch activation. The pilot can burn with minimal gas, while main burners need proper valve and control operation.

Can a Dirty Flame Sensor Cause No Heat?

Yes. A coated or dirty flame sensor may not detect the pilot flame properly and prevent the furnace from opening the main gas valve. Cleaning or replacing the sensor often resolves this issue.

Is It Dangerous To Run A Pilot Light If Burners Won’t Ignite?

Running the pilot alone is not typically dangerous, but underlying issues involving gas flow or safety switches could indicate a larger risk. If the furnace behaves abnormally or a gas smell is present, turn off the unit and contact a professional.

How Long Does A Pilot Light Usually Last?

Pilot lights burn continuously as long as the gas supply is on. Modern systems may use electronic ignition instead of a standing pilot to improve efficiency and safety.

Resources And Next Steps

Homeowners should consult the furnace manual for model-specific diagnostics and error codes. For persistent problems or any gas-safety concerns, contact a licensed HVAC technician or the local gas utility. Keep documentation of repairs and inspections for warranty and resale value.

Key Takeaway: When the pilot light is on but no heat is produced, the issue often lies in control components, sensors, vents, or gas flow to the main burners. Start with simple checks—thermostat, sensor cleaning, and venting—but seek professional service for gas valve, control board, or safety-related repairs.