Gas Furnace Not Igniting: Troubleshooting and Practical Fixes

A gas furnace not igniting can leave a home cold and raise safety concerns. This guide explains common causes, step-by-step troubleshooting, DIY fixes, safety checks, and when to call a professional to restore reliable heating. It focuses on practical, actionable information for homeowners and renters in the U.S.

Common Cause	Quick Symptom	First Action
Pilot Or Hot Surface Igniter	No spark or glow	Inspect igniter for cracks; test continuity
Dirty Flame Sensor	Ignites briefly then shuts off	Clean sensor with fine emery cloth
Thermostat Or Controls	Furnace won’t start	Verify thermostat settings and batteries
Gas Supply Or Valve	Smell of gas or no gas flow	Check gas shutoff and call utility if needed
Airflow/Limit Switch	Blower runs but no ignition	Replace clogged filter; inspect vents

How A Gas Furnace Ignites: Basic Sequence

Understanding the ignition sequence helps pinpoint failures. Modern furnaces use an electronic control board to start a cycle: thermostat calls for heat, inducer motor starts, pressure switch confirms airflow, igniter (hot surface or spark) energizes, gas valve opens, burner lights, flame sensor confirms flame and control allows continuous operation.

Key Components: thermostat, inducer motor, pressure switch, igniter, gas valve, flame sensor, control board, limit switch, blower.

Initial Safety Checks Before Troubleshooting

Safety is paramount. If the smell of gas is present, evacuate immediately and call the gas company or 911.

Turn power to the furnace off at the service switch and shut off the gas if planning internal inspection. Use a flashlight rather than opening panels while power is on if only visual checks are needed.

Check Thermostat And Power Supply

Many ignition issues stem from simple thermostat or power problems. Ensure the thermostat is set to Heat and the temperature is set above room temperature. Replace thermostat batteries and confirm the thermostat is wired correctly for a gas furnace.

Verify that the furnace has power: check the dedicated circuit breaker and the furnace service switch. Look for standby lights or error codes on the control board display.

Igniter Problems: Hot Surface Igniters And Spark Systems

Hot Surface Igniters (HSI) glow orange to light the gas. Common signs of HSI failure include no glow, intermittent glow, or visible cracks. HSIs are fragile and can fail due to age or repeated thermal stress.

Spark ignition systems use electrodes and a high-voltage spark. Symptoms of failure include no spark or weak spark. Visual inspection may reveal carbon buildup or damaged electrodes.

DIY Igniter Tests And Replacement

To test an HSI, unplug power, remove the igniter, and inspect for cracks. An electrical multimeter can test continuity; an open circuit indicates failure. Replacement is typically straightforward and inexpensive but requires careful handling.

For spark systems, check electrode gaps and cleanliness. Replacement or adjustment may require a professional if high-voltage components are involved.

Flame Sensor Problems: Furnace Ignites Then Shuts Off

A dirty or failing flame sensor often causes a furnace to ignite briefly and then shut off. The flame sensor detects combustion; if it doesn’t sense flame, the control closes the gas valve for safety.

Cleaning The Flame Sensor

Turn off power and gas before removing the flame sensor. Remove the sensor and gently clean the metal rod with fine emery cloth or a nonabrasive pad. Reinstall and test the furnace. Cleaning often restores proper operation.

Gas Supply And Gas Valve Issues

If the furnace does not receive gas, burners cannot ignite. Confirm the home’s gas supply: ensure the gas meter and main valve are open, and other gas appliances work. If there is suspicion of a gas leak, leave the building and call the gas utility immediately.

The gas valve itself can fail electrically or mechanically. The control board sends a signal to open the gas valve; if the valve does not open, the ignition stage will fail. Testing the valve usually requires a technician with proper tools and gas-testing equipment.

Pressure Switch And Inducer Motor Problems

The pressure switch ensures the inducer motor creates appropriate vent pressure before ignition. If the inducer motor fails or the vent is blocked, the pressure switch will remain open and prevent ignition.

Inspect the inducer motor for unusual noises and verify the vent is clear of debris or bird nests. A multimeter can test pressure switch continuity during start-up if the homeowner is experienced and comfortable with electrical testing.

Airflow, Filters, And Limit Switches

Restricted airflow from clogged filters or blocked vents can overheat the furnace and trip the limit switch, preventing ignition. Replace filters regularly and ensure supply and return vents are unobstructed.

Limit switches shut down burners to protect the heat exchanger. A faulty switch may falsely detect overheating. Diagnosis often requires professional testing and replacement.

Control Board And Error Codes

Modern furnaces include diagnostic LED codes or digital displays that indicate specific faults. Check the furnace access panel for a sticker explaining blink codes.

Control board failures can disrupt the ignition sequence. Symptoms include no response to thermostat calls, incorrect timing, or random lockouts. Replacing a control board is complex and typically done by a qualified technician.

Reset Procedures And When They Help

Many furnaces have a reset button or will reset by switching power off and on at the breaker. A reset can clear temporary faults and restore normal operation, but repeated resets indicate an unresolved issue.

Use resets sparingly; constant resetting may mask a safety shutdown. If a reset restores function but the problem recurs, schedule a professional inspection.

DIY Troubleshooting Checklist

• Verify Thermostat: Set to Heat, batteries fresh, correct wiring.
• Check Power: Circuit breaker and service switch on.
• Inspect Igniter: Look for cracks; test continuity for HSI.
• Clean Flame Sensor: Remove and gently wipe with fine abrasive.
• Replace Filter: Install a clean air filter to ensure proper airflow.
• Check Vents: Clear intake and exhaust vents of obstructions.
• Listen For Inducer: Confirm it runs before ignition attempt.
• Note Error Codes: Record LED blink patterns for technician reference.

When To Call A Professional

Contact a licensed HVAC technician if there is a gas smell, if the gas valve or control board may be faulty, if pressure switch or inducer motor tests are needed, or if safety devices trip repeatedly.

Professional service is also recommended for complex electrical diagnostics, flame sensor replacement if not comfortable working near gas, and heat exchanger inspection if there are concerns about cracks or carbon monoxide risks.

Safety Precautions And Carbon Monoxide Risks

A malfunctioning furnace can produce carbon monoxide. Install and maintain CO detectors near sleeping areas and on each level. Test detectors monthly and replace batteries at least once a year.

Do not ignore the smell of gas. Evacuate and call emergency services or the gas company. Regular professional maintenance reduces the risk of combustion-related hazards.

Cost Estimates And Typical Repair Times

Costs vary by region and furnace model. Common price ranges: igniter replacement $100–$300, flame sensor cleaning or replacement $75–$200, gas valve replacement $300–$800, control board $300–$900, inducer motor $250–$700.

Minor repairs like cleaning the flame sensor or replacing an igniter often take 30–90 minutes. Major components or diagnostic work can take 2–4 hours or longer, especially if part ordering is required.

Preventive Maintenance To Avoid Ignition Failures

Schedule annual professional tune-ups before heating season. Tasks should include burner inspection, gas pressure check, flame sensor cleaning, igniter testing, vent and flue inspection, and heat exchanger examination.

Homeowners should change filters monthly to quarterly, keep vents clear, and monitor CO detectors. Promptly address odd noises, frequent cycling, or reduced heat output.

Frequently Asked Questions

Why Does The Furnace Click But Not Ignite?

Clicking may be the igniter attempting to spark or the gas valve cycling. If the igniter clicks without ignition, the issue may be the gas valve, blocked gas supply, or no spark/failed HSI.

Can A Dirty Filter Prevent Ignition?

Yes. A severely clogged filter can restrict airflow, trigger high-limit switches, and prevent the ignition sequence from completing.

Is It Safe To Try DIY Fixes?

Simple tasks like replacing filters, cleaning the flame sensor, and checking thermostat settings are generally safe. Electrical, gas valve, or control board work should be left to licensed technicians.

Resources And References

For official safety guidelines, consult the U.S. Consumer Product Safety Commission and local utility company resources. Manufacturer manuals provide model-specific diagnostic and error code information.

When in doubt about gas-related issues or carbon monoxide risks, contact a licensed HVAC professional or local utility for immediate assistance.

Keywords: Gas Furnace Not Igniting, furnace igniter, flame sensor cleaning, gas valve, thermostat troubleshooting, furnace safety, carbon monoxide detector.