Furnace Blower Won’t Turn Off: Causes, Diagnostics, and Fixes

The furnace blower won’t turn off can signal anything from a simple thermostat setting issue to a failing control board. This article explains common causes, step-by-step diagnostics, safety checks, DIY fixes, and when to call a professional so homeowners can identify and resolve the problem efficiently.

Symptom	Likely Cause	Quick Fix
Blower runs continuously	Fan limit switch set to ON or thermostat fan set to ON	Change thermostat fan setting to AUTO
Runs after furnace shuts off	Delay relay or post-purge feature	Normal behavior; check timing
Won’t stop even with power off	Stuck relay or shorted control	Turn off power and call technician

How The Furnace Blower System Works

The furnace blower circulates heated or cooled air through ductwork using a motor and fan assembly. A thermostat calls for heat or cooling, energizing the furnace’s control board. The control board engages a relay that supplies 120V (or appropriate voltage) to the blower motor when circulating air is required.

A fan relay, fan limit switch, or the thermostat’s fan setting determines whether the blower runs continuously or only during heating/cooling cycles. Modern systems may also include an electronic control board with timed or staged blower control features.

Common Causes When The Furnace Blower Won’t Turn Off

Thermostat Fan Set To ON

The thermostat fan setting controls whether the blower runs continuously (ON) or only during calls for heat/cool (AUTO). A common oversight is leaving the fan on.

Faulty Thermostat Or Wiring

A malfunctioning thermostat or shorted wiring to the G (fan) terminal can keep the blower energized. Corroded terminals or loose connections can create persistent calls for fan operation.

Stuck Fan Relay Or Contactor

The relay or contactor on the control board that switches power to the blower motor can weld or stick closed, causing continuous operation. Mechanical wear or power surges can damage these components.

Bad Control Board Or Relay Driver

An electronic control board managing blower functions can fail, sending continuous power to the motor. Failed relay drivers or stuck solid-state relays produce similar symptoms.

Fan Limit Switch Malfunction

Some furnaces use a fan limit switch to stop or start the blower based on heat exchanger temperature. A malfunctioning switch or sensor can misread temperatures and leave the blower running.

Post-Purge Or Delay Feature

Many furnaces run the blower for a short time after the burner shuts off (post-purge) to extract residual heat. Extended or abnormal post-purge may appear as the blower won’t turn off but might be normal if timed correctly.

Stuck Relay Inside The Blower Motor (ECM/PSC Motors)

Blower motors with internal relays or electronically commutated motors (ECM) can develop internal faults that prevent shutoff. Older PSC motors use external relays that can stick.

Safety Precautions Before Troubleshooting

Always prioritize safety. Turn off power at the furnace switch and the breaker before inspecting internal components. Use a non-contact voltage tester if available. If any step requires disassembly beyond the access panels or exposes live wiring, stop and call a licensed HVAC technician.

Step-By-Step Diagnostic Checklist

1. Check Thermostat Fan Setting

Set the thermostat fan to AUTO and observe the blower. If the blower stops, the issue is resolved. If it continues, proceed to the next step.

2. Cycle Power And Observe Behavior

Turn the furnace power off for 30 seconds at the switch or breaker, then restore power. If the blower runs continuously after power restoration, the problem is likely electrical or control-related.

3. Inspect Thermostat Wiring

Remove the thermostat cover and look for wiring on the G terminal. Ensure wires are not touching other terminals or the wall plate and that the connection is secure. Tighten screws and reseat wires if needed.

4. Test The G Wire With A Jumper

With the furnace powered and thermostat off, briefly short the G terminal to R at the furnace or thermostat to simulate a fan call. If the blower responds only while shorted, thermostat/wiring may be fine. Continuous operation without removing the jumper indicates a stuck relay or control board issue.

5. Locate And Inspect The Fan Relay/Contactor

Identify the relay on the control board or a separate fan relay box. Look for signs of welding, burning, or mechanical sticking. Tap the relay gently after de-energizing to see if the contacts free up—do not attempt this if unsure.

6. Measure Voltage To The Blower Motor

With proper safety and tools, measure voltage at the motor terminals when the blower is running. Continuous 120V (or rated voltage) indicates a closed relay; zero voltage with blower running suggests internal motor issue or wiring fault.

7. Inspect Fan Limit Switch And Temperature Sensors

Check the fan limit switch or temperature sensor for obvious damage or loose connections. Test with an ohmmeter if a schematic and switch ratings are available.

8. Test Motor Capacitor And Motor Health

A failing motor capacitor can cause erratic motor behavior but typically prevents starting rather than causing continuous run. Still, inspect the capacitor for bulging or leaking and test capacitance if equipped to do so safely.

DIY Fixes Homeowners Can Try

Reset Thermostat Settings

Reset the thermostat to factory defaults or remove batteries for a minute. Reconfigure fan settings to AUTO. This often resolves software glitches in smart thermostats.

Clean And Tighten Thermostat Wiring

Turn off power, clean terminal contacts, and ensure secure wiring on the thermostat and control board. Replace frayed wires if present. Use correct gauge wiring for longer runs.

Replace A Faulty Mechanical Fan Relay

If a separate plug-in fan relay is visibly damaged, replacing it can be a straightforward repair. Ensure matching coil voltage and contact ratings when sourcing a replacement relay.

Replace The Fan Limit Switch

Replace a damaged mechanical fan limit switch with a compatible part. This task is within reach for DIYers comfortable accessing furnace panels and following wiring diagrams.

When To Call An HVAC Technician

Call a professional if the blower continues to run after basic checks, if wiring or control board replacement is required, or if the problem involves gas or combustion components. A licensed technician will safely diagnose control boards, replace relays, and test internal motor components.

Call Immediately if the blower runs even with the furnace power breaker off—this suggests a wiring fault or feedback that may present shock risk.

Costs And Typical Repairs

Thermostat issues or wiring repairs typically cost under $200 when performed by a technician. Replacing a fan relay or limit switch often ranges from $150–$350 including parts and labor. Control board replacement can cost $400–$900 depending on model and complexity. Emergency service increases prices.

Preventive Maintenance To Avoid Future Problems

Schedule annual furnace tune-ups to inspect relays, control boards, motors, and sensors. Replace air filters every 1–3 months to reduce motor strain. Keep the thermostat in a clean, dry location away from drafts and direct sunlight to prevent false commands.

Document wiring changes and label thermostat wires during servicing. Consider installing a surge protector for HVAC equipment to protect control electronics from power spikes.

Troubleshooting Quick Reference

Symptom	Likely Fix
Blower Runs Continuously After Setting Fan To AUTO	Check thermostat wiring, replace fan relay or control board
Blower Runs For Several Minutes After Burner Shuts Off	Normal post-purge; confirm timing with manual or tech
Blower Runs With Furnace Power Off	Turn off main breaker and call electrician/HVAC

Common Myths And Misconceptions

Myth: “A dirty filter causes the blower to run nonstop.” While a clogged filter increases blower strain and runtime, it rarely forces continuous operation; underlying control issues are more likely.

Myth: “Turning thermostat off will stop the blower.” If the cause is a stuck relay or control board fault, the blower can remain powered despite thermostat commands.

Key Takeaways

When the furnace blower won’t turn off, the most likely causes are thermostat fan settings, stuck relays, faulty control boards, or wiring shorts. Simple fixes include changing fan setting to AUTO, checking thermostat wiring, and resetting the thermostat. Complex electrical or control board failures should be handled by licensed technicians.

Prioritize safety: always cut power before working inside the furnace and call a professional for live electrical diagnostics, gas-related issues, or if the blower continues to run with power removed.

For persistent problems, maintain a service record and consider scheduled maintenance to reduce future failures and extend system life.