Furnace Blower Comes on but Won’t Ignite: Causes, Troubleshooting, and Fixes

The furnace blower running while the burners fail to ignite is a common and concerning HVAC issue that leaves homes cold and homeowners confused. This article explains the most likely causes, step-by-step diagnostic checks, safety precautions, repair options, estimated costs, and preventive maintenance to restore reliable heat. Critical safety reminders and guidance on when to call a licensed technician are included.

Symptom	Likely Causes	Typical Fix
Blower runs; no ignition	Thermostat issue, flame sensor, igniter, gas supply, control board	Reset/replace component, clear sensor, restore gas, call pro
Blower cycles on then off	Pressure switch, limit switch, blocked vent	Clean vents, replace switch
Blower runs continuously	Fan limit stuck, thermostat fan setting	Adjust thermostat, replace limit control

How A Gas Furnace Ignition Sequence Normally Works

Understanding the standard ignition sequence helps pinpoint failures. When the thermostat calls for heat, the furnace runs a safety check, starts the inducer motor, verifies vent pressure, opens the gas valve, energizes the igniter (hot surface or spark), and detects flame with a flame sensor or electrode. Once flame is confirmed, the blower circulates warm air through the ductwork. Any interruption in that chain can result in the blower running without ignition.

Common Reasons The Blower Runs But Ignition Fails

Several components may cause the blower-to-ignition disconnect. The most frequent culprits are a faulty igniter, dirty flame sensor, no gas supply, malfunctioning gas valve, failed pressure switch, broken thermostat, or a problematic control board. Age, lack of maintenance, and electrical issues also contribute.

Faulty Igniter (Hot Surface Or Spark)

The igniter is responsible for creating heat or a spark to light the gas. Hot surface igniters degrade over time and will glow but may not reach ignition temperature. Spark igniters can fail electrically. A cracked or weak igniter often prevents ignition while allowing the blower to run.

Dirty Or Failing Flame Sensor

The flame sensor confirms combustion. When it’s coated with carbon or corrosion, it may not detect flame and will shut off the gas valve shortly after ignition attempts, leaving the blower running. Cleaning or replacing the sensor typically resolves this issue.

No Gas Supply Or Closed Gas Valve

If the gas supply is interrupted—by a closed shutoff valve, gas company outage, or regulator problem—the burners won’t ignite. The furnace may try to fire but cannot, while the blower still runs as part of the startup sequence.

Malfunctioning Pressure Switch Or Inducer Motor

The pressure switch confirms proper venting by sensing inducer draft. If the inducer motor fails or the pressure switch is stuck open/closed, the control board will prevent ignition for safety reasons. Blocked venting or a cracked inducer can produce the same effect.

Control Board Or Relay Failures

The furnace control board orchestrates the ignition process. A failed board or a bad relay may energize the blower circuit while not sending the correct signals to the gas valve or igniter, causing the blower to run without ignition.

Thermostat Or Wiring Problems

A thermostat that incorrectly signals fan-only mode, or wiring faults between the thermostat and furnace, can create scenarios where the blower runs but the heat call is not properly transmitted. Incorrect configuration of the fan setting can also run the blower continuously.

Safety First: What To Do Immediately

If the blower is running but burners aren’t igniting, safety precautions are essential. Shut off the furnace and gas supply if there is any smell of gas. Open windows, evacuate if the odor is strong, and call the gas company or 911. If no gas smell is present, switch off power to the furnace before inspecting components to avoid electric shock or damage.

Step-By-Step Troubleshooting Guide

Perform basic checks in this order to avoid unnecessary repairs and to isolate the problem safely. Only attempt visual or simple tasks if comfortable with basic home maintenance. For most tasks involving gas or open panels, consider a licensed HVAC technician.

1. Check The Thermostat: Ensure it is set to heat and the fan setting is not set to “On” (which keeps the blower running constantly). Replace batteries and confirm the temperature setpoint is above room temperature.
2. Inspect Filters And Vents: Replace dirty air filters and clear obstructed vents. Restricted airflow can trip safety limits or affect ignition cycles.
3. Listen For Ignition Attempts: Cycle the furnace and listen for the inducer motor, igniter glow or spark, and gas valve click. Note any sequences that are missing.
4. Check For Gas Supply: Verify the gas shutoff valve near the furnace is open and other gas appliances (stove, water heater) are operating. Contact the gas company if supply is suspected off.
5. Examine The Igniter: For hot surface igniters, look for visible cracks off the orange glow. Do not touch the igniter with bare hands. Replace if cracked or not glowing.
6. Clean The Flame Sensor: Remove the sensor and gently clean the metal rod with fine-grit sandpaper or emery cloth to restore contact. Reinstall and retest the furnace.
7. Inspect The Inducer And Pressure Switch: Ensure the inducer runs and pressure tubing isn’t blocked or disconnected. A multimeter can test the pressure switch continuity if comfortable.
8. Reset The System: Some control boards require a manual reset after multiple failed ignition attempts. Power-cycle the furnace by turning off the breaker for 30 seconds and then turning it back on.

When To Replace Parts

Some parts degrade predictably and have typical lifespans. Igniters and flame sensors are common homeowner-serviceable items, but replacement should follow manufacturer instructions and local codes. Control boards, gas valves, and inducer motors usually require professional diagnosis and installation.

Igniter Replacement

Hot surface igniters typically last 3–10 years. Replace the igniter if visibly cracked, electrically open, or no longer glowing. Use OEM parts to match voltage and resistance specifications.

Flame Sensor Replacement

Flame sensors are inexpensive and often replaced after cleaning fails. If corrosion persists or the sensor is damaged, replacement restores reliable flame detection.

Control Board And Gas Valve

Faulty control boards or gas valves should be replaced by a licensed technician due to safety and calibration requirements. These parts are pricier and require precise installation and testing.

Tools And Tests A Technician Uses

A qualified HVAC technician uses specialized tools and tests to confirm causes, including multimeters, manometers, gas pressure gauges, combustion analyzers, and leak detectors. They will examine ignition voltages, flame rectification current, gas manifold pressure, and exhaust draft to diagnose complex failures accurately.

Estimated Costs For Common Repairs

Costs vary by region and furnace model. Typical ranges in the U.S. are provided for planning purposes.

Repair	Estimated Parts Cost	Labor Range
Clean Flame Sensor	$10–$40	$50–$120
Igniter Replacement	$30–$120	$100–$250
Pressure Switch/Inducer Motor	$50–$400	$150–$450
Gas Valve Replacement	$150–$450	$200–$600
Control Board Replacement	$150–$600	$200–$700

When To Call A Professional

A licensed HVAC technician should be called when gas components, control boards, or sealed system elements are suspected to be faulty, when there is a persistent gas odor, or when the homeowner is uncomfortable performing basic checks. Technicians ensure safe handling of gas lines, combustion adjustments, and code-compliant repairs.

Preventive Maintenance To Avoid Future Failures

Regular maintenance reduces ignition failures and extends furnace life. Recommended tasks include annual professional tune-ups, replacing air filters every 1–3 months, cleaning flame sensors, keeping vents clear, and ensuring proper thermostat operation. A maintained system is less likely to experience unexpected ignition issues.

Common Mistakes To Avoid

• Attempting major repairs on gas valves or control boards without licensing and proper training.
• Using abrasive chemicals on flame sensors or igniters that can damage sensitive surfaces.
• Ignoring a persistent gas smell; always treat it as an emergency and call professionals.
• Overlooking simple fixes like thermostat settings or clogged filters before replacing expensive parts.

How To Choose The Right HVAC Technician

Choose licensed, insured HVAC professionals with good local reviews, clear pricing, and experience with the furnace brand. Ask for references, warranties on parts and labor, and proof of proper gas handling credentials. A reputable technician will provide a clear diagnostic report and multiple repair options when appropriate.

Key Takeaways And Fast Checklist

Key Points: The blower running without ignition usually points to igniter, flame sensor, gas supply, pressure switch, or control board issues. Start with safe, simple checks like thermostat settings and air filters, then proceed to cleaning the flame sensor and inspecting the igniter. Call a licensed technician for gas-related or complex electrical problems.

1. Check thermostat and fan setting.
2. Replace/clean air filter.
3. Listen for inducer, igniter, gas valve clicks.
4. Verify gas supply and smell for leaks.
5. Clean or replace flame sensor.
6. Replace igniter if cracked or not glowing.
7. Contact an HVAC pro for control board, gas valve, or inducer issues.

Additional Resources

Homeowners can consult furnace manuals, manufacturer websites, and reputable HVAC associations such as the Air Conditioning, Heating, and Refrigeration Institute (AHRI) or local utility conservation programs for maintenance tips and certified contractor directories. Always prioritize safety and code compliance when dealing with gas appliances.