Furnace Blower Won’t Shut Off and No Heat: Causes and Fixes

The furnace blower running constantly while the system fails to produce heat is a common, frustrating problem for homeowners. This article explains likely causes, step-by-step troubleshooting, safety precautions, and practical fixes to help restore normal operation. It emphasizes diagnostic order and when to call a licensed technician.

Symptom	Possible Causes	Quick Action
Blower Runs Continuously	Thermostat Fan Set To On, Stuck Relay, Control Board Fault, Shorted Wiring	Check thermostat setting, power cycle furnace, inspect control board
No Heat	Ignition Failure, Gas Supply, Clogged Filter, Limit Switch Tripped	Confirm gas/electric supply, replace filter, check error codes
Blower Runs But No Heat	Blower Relay Stuck Closed, Limit Switch Bypassed, Flame Sensor Dirty	Test relay, inspect flame sensor, schedule professional service

How The Furnace And Blower Interact

A gas or electric furnace uses a thermostat to call for heat and for the blower fan. When the thermostat calls for heat, the furnace ignites (or energizes electric elements), the heat exchanger warms, and the blower kicks on at the correct time to distribute warm air. If the blower runs continuously but there is no heat, the fault may be on the control/relay that manages the motor or in the heating sequence itself.

Common Causes When The Furnace Blower Won’t Shut Off And There’s No Heat

Thermostat Fan Setting Or Malfunction

A thermostat set to Fan: On runs the blower continuously regardless of heat call. A malfunctioning thermostat or wiring short can also keep the fan energized even when the thermostat is set to Auto.

Stuck Blower Relay Or Contactor

The blower motor is switched by a relay or contactor on the control board. A mechanical or electrical failure can leave the relay stuck closed, supplying constant power to the motor while the heating circuit may not be activated.

Faulty Control Board

The furnace control board coordinates ignition, blower timing, and safety interlocks. A failed board can energize the blower continuously or fail to initiate the heating cycle, producing the symptom of blower on with no heat.

Limit Switch Or High-Temperature Safety Trip

The limit switch senses furnace cabinet temperature. If it is failed, shorted, or tripped, it may prevent burner operation while allowing the blower to run, or the switch wiring can cause continuous blower operation.

Pressure Switch Or Inducer Issues

Modern furnaces use a pressure switch and inducer motor for safe combustion. If the inducer or pressure switch fails the furnace may not ignite, though blowers (particularly the circulation fan) may still run.

Ignition, Gas Valve, Or Pilot Problems

If the ignition sequence fails—failed ignitor, clogged pilot, or closed gas valve—the furnace will not heat. The blower may continue running from a thermostat fan call or a relay fault while burners remain off.

Dirty Flame Sensor Or Ignition Rod

A dirty flame sensor can extinguish the gas valve shortly after ignition, causing no sustained heat. The blower may run between ignition attempts or remain on if wiring faults exist.

Shorted Wiring Or Control Circuit Ground

Broken, pinched, or shorted wires between the thermostat, control board, or blower motor can create unintended signals that keep the motor energized while disabling heat circuits.

Step-By-Step Troubleshooting Guide

Safety First

Before inspecting a furnace, turn off power at the service switch and, for gas units, shut off the gas if instructed by a professional. Never touch live terminals or internal components unless qualified. If there is any gas smell, evacuate and call the gas company.

Verify Thermostat Settings And Power

• Confirm the thermostat is set to Heat and Fan is set to Auto, not On.
• Replace thermostat batteries and reboot the thermostat by removing power for 30 seconds.
• If a smart thermostat is used, check for software errors or recent updates.

Check Filters, Vents, And Ducts

A clogged filter or blocked return can cause overheating trips or limit switch activation. Replace dirty filters and ensure returns are open. Though filter issues usually reduce heat rather than stop it, they should be ruled out early.

Inspect Control Board And Error Codes

Open the furnace door and observe the control board LED(s). Many boards flash diagnostic codes indicating ignition, pressure switch, or limit switch faults. Record and reference the manufacturer error list before proceeding.

Listen For Relays And Inducer Operation

When the thermostat calls for heat, note whether the inducer motor runs and whether the ignitor glows or spark sequence begins. If the blower runs constantly while the inducer and ignition never start, suspect a blower relay/control board fault.

Test The Blower Relay And Motor

• With power off, inspect the relay for welded contacts or burn marks.
• Measure voltage at the blower motor terminals when the blower is running to determine if the motor receives continuous 120/240V supply.
• If the relay is stuck closed, replacing the relay or control board may be required.

Evaluate Limit And Pressure Switches

Check continuity of limit switches and pressure switches when the system is off. A tripped limit may be reset by turning power off for several minutes, but persistent trips indicate airflow or heat-exchanger issues requiring professional diagnosis.

DIY Fixes Homeowners Can Safely Attempt

• Set Thermostat Fan To Auto: Change the Fan setting and observe if blower stops. This simple step often fixes continuous blower runs.
• Replace Dirty Filter: Install a new furnace filter and check airflow and limit behavior.
• Clean Flame Sensor: For gas furnaces, power down, remove the flame sensor, clean lightly with fine emery or steel wool, reinstall and test ignition sequence.
• Power Cycle Furnace: Turn off power at the furnace switch or breaker for 60 seconds to reset control board logic.
• Inspect For Visible Wiring Damage: With power off, look for burnt wires or melted connectors and document issues for the technician.

When To Call A Professional Technician

A technician should be called if the homeowner is uncomfortable working with gas or electrical components, if diagnostic LEDs indicate ignition or pressure switch failures, or if relay/control board replacement is necessary. Persistent tripping of safety devices, possible cracked heat exchanger, or gas valve issues require licensed service.

Immediate professional help is required if there is a gas odor, visible flame rollout, loud popping noises, or carbon monoxide concerns.

Estimated Repair Costs And Timeframes

Issue	Typical DIY Cost	Professional Service Cost	Time
Thermostat Reset/Replacement	$0–$150	$100–$300 Installed	30–90 Minutes
Filter Replacement	$10–$60	$20–$80	5–15 Minutes
Flame Sensor Clean/Replace	$0–$40	$80–$200	30–60 Minutes
Blower Relay Or Motor Repair	Not Recommended DIY	$150–$700	1–3 Hours
Control Board Replacement	Not Recommended DIY	$300–$1,200	1–3 Hours
Ignitor Or Gas Valve Repair	Not Recommended DIY	$150–$600	1–3 Hours

Preventive Maintenance To Avoid Future Problems

• Replace filters every 1–3 months depending on occupancy and pets.
• Schedule annual professional furnace service before cold seasons.
• Keep returns and supply vents unobstructed and ensure adequate combustion air.
• Install a carbon monoxide detector near bedrooms and the furnace room.
• Upgrade to a quality thermostat and check wiring during service visits.

Safety Tips And Carbon Monoxide Awareness

Continuous blower operation with no heat can mask combustion issues. Ensure working CO detectors and perform periodic CO testing during professional maintenance. Never disable safety switches to force heat, and never attempt gas valve repairs without certification.

Key Diagnostic Checklist

1. Confirm thermostat Fan set to Auto and thermostat batteries/power are good.
2. Replace the air filter and verify return airflow.
3. Observe control board LEDs and note error codes.
4. Listen for inducer and ignition; record what runs and what does not.
5. Power cycle the furnace to reset relays and control logic.
6. Call a licensed HVAC technician if ignition, pressure switch, control board, gas valve, or heat exchanger issues are suspected.

Additional Resources

For model-specific diagnostics, consult the furnace manufacturer’s service manual and code lists. For safety questions or suspected gas leaks, contact the local utility or a licensed HVAC professional immediately.

By following the troubleshooting order—thermostat, filters/airflow, control board and relays, then ignition and safety devices—homeowners and technicians can quickly narrow down why the furnace blower won’t shut off yet produces no heat and take appropriate corrective action.