Skip to main content
True Trade Path

Diagnostic guide

Furnace Blower Won't Stop Running

The furnace blower runs continuously even when heating or cooling is not called — find out why and what it costs to fix.

System
Heating
Causes covered
6
You can check
1 without a technician

Stop and read this first

  • A blower that runs continuously with no heat output and a hot cabinet may indicate a heat exchanger problem — shut the system off and call a technician.
  • Do not disable safety switches to stop the blower.
  • Continuous operation increases electricity costs — a blower motor uses 300 to 800 watts.

Work through these in order

Ordered from most to least likely. Working top down means you rule out the cheap, common causes before paying anyone to look at the expensive, rare ones.

  1. Thermostat fan set to On

    CommonYou can check
    How to check
    The fan setting on the thermostat is On rather than Auto. Blower runs whether or not heat or cool is active.
    The fix
    Set the fan to Auto. This is the most common cause and costs nothing to fix.

  2. Stuck fan relay on the control board

    CommonNeeds a pro
    How to check
    Fan runs with the thermostat set to Auto and no heating or cooling call active. Relay clicks but does not release.
    The fix
    A technician replaces the relay or control board — typically $200 to $600 depending on the board.

  3. Short in the thermostat wiring

    PossibleNeeds a pro
    How to check
    Fan runs even with the thermostat disconnected or set to Off. Problem started after thermostat replacement.
    The fix
    A technician traces the low-voltage wiring. A short between the G (fan) and R (power) terminals keeps the blower energized.

  4. Limit switch stuck closed

    PossibleNeeds a pro
    How to check
    Blower runs continuously and the furnace cabinet feels hot. Furnace may have been short cycling before this started.
    The fix
    A technician tests and replaces the limit switch. If the switch failed from repeated overheating, also replace the filter and check airflow.

  5. Failed blower motor limit (internal)

    PossibleNeeds a pro
    How to check
    Blower runs at one speed only and does not respond to thermostat commands.
    The fix
    A technician diagnoses the motor or its integrated control module. Blower motor replacement runs $450 to $1,400.

  6. Incorrect wiring after thermostat or board replacement

    PossibleNeeds a pro
    How to check
    Problem began immediately after a DIY thermostat install or recent service.
    The fix
    Verify wiring against the manufacturer diagram. A service call to correct wiring runs $150 to $300.

Once you know what it is

Step one

Know what it should cost

Get the range before you call, so the first number you hear is not the only number you have.

Step two

Decide repair or replace

On equipment near end of life, the repair is usually the first of several.

Step three

Check the quote

Compare against the typical range and confirm the terms before you sign.

Furnace Replacement cost guideRepair or replace calculatorEstimate repair costAnalyze a quote

What the fix costs

Tools for this decision

Other symptoms