Furnace Lights but Blower Won’t Start: Causes and Fixes

The furnace lights but the blower doesn’t come on is a common HVAC issue that can leave a home cold and homeowners frustrated. This article explains likely causes, step-by-step diagnostics, safe troubleshooting, expected repair costs, and when to call a professional to restore reliable heating. Key search phrase: Furnace Lights But Blower Doesn’t Come On

Symptom	Possible Causes	Quick Fixes
Burners Ignite But No Airflow	Blower Motor Failure, Capacitor, Control Board	Reset Power, Check Breaker, Inspect Capacitor
Intermittent Blower Operation	Thermostat, Limit Switch, Relay	Test Thermostat, Replace Limit Switch
Blower Runs Then Stops	Overheating, Restricted Vent, Dirty Filter	Replace Filter, Clear Vents, Check Flame Sensor

How Furnaces Normally Operate

A gas furnace sequence begins with a thermostat calling for heat, an ignition event lighting the burners, heat exchange warming, and a blower motor circulating warm air. This sequence depends on control signals from the thermostat, the furnace control board, safety switches, and the blower motor assembly.

When the burners light but the blower won’t come on, the ignition and gas delivery are working while the airflow control path is failing. Diagnosing this separation narrows the issue to the fan circuit, controls, or safety devices.

Common Causes When Furnace Lights But Blower Doesn’t Come On

Blower Motor Failure

The blower motor may seize or lose windings due to age, wear, or electrical damage. A motor that hums without turning or draws excessive current often needs replacement.

Faulty Run Capacitor

The run capacitor provides the motor’s startup and running torque. A weak or failed capacitor can prevent the motor from starting while leaving the rest of the furnace functioning.

Blower Relay Or Control Board Problems

The furnace control board or a relay controls voltage to the blower motor. Failed relays or burnt traces on the control board can stop the blower from receiving power.

Thermostat Or Wiring Issues

If the thermostat fails to send the fan-on or heat call signal, or thermostat wiring is loose or damaged, the blower may not be commanded to run even though the burners ignite.

Limit Switch Or Safety Switch Tripped

High-limit switches, rollouts, or other safety devices disable the blower to prevent damage. A tripped limit due to overheating or a stuck switch can stop fan operation until reset or replacement.

Clogged Air Filter Or Blocked Return

Severely restricted airflow causes overheating and may trigger safety cutoffs that prevent the blower from running or will cycle it off prematurely.

Motor Overload Or Tripped Breaker

A dedicated blower breaker or motor overload protector can trip. Resetting may restore operation temporarily but indicates underlying issues such as motor problems or short circuits.

Step-By-Step Troubleshooting Guide

Safety First

Turn off electrical power to the furnace at the breaker before inspecting internal components. For gas furnaces, turn off the gas supply if dismantling components or if a gas smell is present. If uncomfortable with electrical or gas work, contact a licensed technician.

Verify Thermostat And Settings

Set the thermostat to heat and raise the setpoint above room temperature. Confirm fan setting is on Auto or On depending on the test. Replace thermostat batteries and, if available, try a second thermostat to rule out control issues.

Check Power And Breakers

Inspect the furnace circuit breaker and any secondary switches. Reset any tripped breakers. Confirm 120/240V power at the furnace disconnect and the blower motor’s power connection using a multimeter if qualified.

Inspect Air Filter And Vents

Replace a dirty filter. Ensure supply and return vents are open and unobstructed. Restricted airflow can trip limit switches that prevent the blower from running.

Listen And Observe The Blower Motor

With power on and burners lit, listen for motor hum or vibration. If the motor hums but doesn’t spin, the start capacitor or motor bearings may be at fault. No sound likely indicates no power to the motor or a control failure.

Test The Run Capacitor

A visible bulge or leakage on the capacitor indicates failure. For those qualified, discharge the capacitor safely and test capacitance with a meter. Replace if outside specified tolerance.

Inspect Control Board And Relays

Look for burned components, scorched traces, or overheated solder joints. Check whether the fan relay clicks when a heat call is made. A silent relay on a verified heat call suggests a relay or control board issue.

Check Limit Switches And Safety Devices

Locate the high-limit switch and reset or test continuity while cool. Replace a switch that fails continuity or repeatedly trips. Address root causes of overheating such as blocked vents or dirty heat exchangers.

Examine Wiring And Connectors

Inspect wiring for loose terminals, corrosion, fraying, or rodent damage. Secure or replace damaged connectors. Poor wiring can interrupt the fan signal despite burners igniting.

When To Replace Parts Versus Repair

If the motor shows mechanical failure, replacement is usually required. A new motor may cost $200–$800 installed depending on furnace type. Capacitors are inexpensive ($10–$50) and are often replaced as a first step when motors won’t start.

Control board replacement ranges from $200–$600 installed. Thermostat or relay repairs are generally lower cost. Balance repair expense against furnace age; replacement may be sensible for units older than 15 years.

Safety Risks And Precautions

A malfunctioning blower can produce unsafe combustion conditions if heat is trapped or if exhaust ventilation is compromised. Carbon monoxide risk increases with blocked flues or cracked heat exchangers.

Always shut off power before working inside the furnace. For gas appliances, avoid amateur attempts at gas valve or burner adjustments. Use a licensed HVAC technician for combustion-related issues and CO testing.

Expected Repair Costs And Timeframes

Issue	Typical DIY Cost	Typical Professional Cost	Repair Time
Replace Air Filter	$10–$50	$25–$75	15–30 Minutes
Replace Capacitor	$15–$60	$75–$150	30–60 Minutes
Replace Blower Motor	$120–$500 (part)	$250–$800 (installed)	1–3 Hours
Control Board Replacement	Not Recommended	$200–$600	1–2 Hours

Preventive Maintenance To Avoid Future Failures

Replace filters every 1–3 months depending on usage and filter type. Schedule annual professional maintenance to clean burners, inspect heat exchanger, test controls, lubricate motors, and verify safe operation.

Keep returns and vents clear, maintain proper thermostat calibration, and ensure the area around the furnace remains clean and dry. Early detection of strange noises or reduced airflow can prevent major breakdowns.

When To Call A Professional

Call a licensed HVAC technician if troubleshooting identifies electrical failures, repeated limit trips, suspected heat exchanger cracks, gas valve problems, or if the blower motor must be replaced. Professionals can perform safe live-voltage diagnostics and combustion analysis.

If there is any odor of gas or signs of carbon monoxide, evacuate the home and call emergency services and an HVAC technician immediately.

Frequently Asked Questions

Why Do The Burners Light But The Fan Stays Off?

Because ignition and gas systems operate separately from the fan circuit. The furnace can light burners using the control board while the fan receives power through a different relay, capacitor, or motor connection that may have failed.

Can A Bad Capacitor Cause This Issue?

Yes. A weakened or failed run capacitor often prevents the blower from starting while other systems appear normal. Capacitor replacement is a common and affordable fix.

Is It Safe To Run A Furnace Without The Blower?

No. Running burners without proper airflow can overheat components and trip safety devices; prolonged operation risks damage and potential safety hazards. Avoid running until the blower issue is resolved.

Useful Resources And Tools

• ASHRAE — Technical standards and HVAC guidance
• Energy Star — Efficient heating recommendations
• Multimeter for voltage and continuity checks
• Capacitor tester or meter with capacitance setting

Key Takeaways

When A Furnace Lights But The Blower Doesn’t Come On, the problem is typically within the fan circuit: blower motor, capacitor, control board, relays, safety switches, thermostat, or airflow restrictions. Safe, methodical troubleshooting often identifies the cause. Replace inexpensive components like capacitors early; involve professionals for electrical, gas, or combustion-related repairs.