How To Repair Gas Furnaces


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  • Troubleshooting Gas Furnaces and Gas Heaters

    There are many potential reasons that a furnace might not be working, but fortunately, many of the solutions are as easy as pressing a button. Use this chart to fix a wide range of conditions that prevent your furnace from working.

    Problem  Possible cause  Solution 
    Furnace won't run

    1. No power.

    1. Check for blown fuses or tripped
        circuit breakers at main entrance
        panel, at separate entrance panel,
        and on or in furnace; restore circuit.
      2. Switch off.

    2. Turn on separate power switch
        on or near furnace

      3. Motor overload.

    3. Wait 30 minutes; press reset
        button. Repeat if necessary.

      4. Pilot light out.
    4. Relight pilot.
      5. No gas.

    5. Make sure gas valve to furnace is
        fully open

    Not enough heat
    1. Thermostat set
        too low.

    1. Raise thermostat setting 5°.

      2. Filter dirty.
    2. Clean or replace filter.
      3. Blower clogged.
    3. Clean blower assembly.
      4. Registers closed
        or blocked.
    4. Make sure all registers are open;
        make sure they are not blocked
        by tugs, drapes or furniture.

      5. System out of balance.
    5. Balance system. 
      6. Blower belt loose
        or broken.

    6. Adjust or replace belt.
      7. Burner dirty.  7. Call a professional.
    Pilot won't light
    1. Pilot opening blocked.
    1. Clean pilot opening.
      2. No gas.


    2. Make sure pilot light button
        is fully depressed; make sure
        gas valve to furnace is fully open.

    Pilot won't
    stay lit

    1. Loose or faulty

    1. Tighten thermocouple nut
        slightly, if no results, replace

      2. Pilot flame set
        too low.

    2. Adjust pilot so flame is
        about 2 inches long.

      3. Electric pilot faulty.
    3. Call a professional.
    Furnace turns
    on and off

    1. Filter dirty.
    2. Motor and/or blower
    1. Clean or replace filter.
    . If motor and blower have
        oil ports, lubricate.

    Blower won't
    stop running

    1. Blower control
        set wrong.
    1. Reset thermostat from
        ON to AUTO.
      2. Limit switch set wrong.

    2. Reset limit switch
        for stop-start cycling.

      3. Limit control
        needs adjustment.

    3. Call a professional.
    Furnace noisy

    1. Access panels loose.

    1. Mount and fasten access
        panels correctly.
      2. Belts sticking, worn,
        or damaged.

    2. Spray squeaking drive belts
        with belt dressing; replace worn
        or damaged belts.

      3. Blower belts
        too loose or too tight.
    3. Adjust belt.

      4. Motor and/or
        blower needs

    4. If motor and blower
        have oil ports, lubricate.

    5. Burner dirty.
    5. Call a professional.

    The pilot light is crucial to the operation of the furnace. Learn how to light a pilot light on the next pae.

    For more articles on home repair, check out the following links.

    • Furnace Maintenance: Save yourself time and money by learning the steps to keep your furnace in prime condition.
    • Major Appliance Repair: If the furnace isn't the only thing in your house on the fritz, you can learn how to fix other machines in this article.
    • Small Appliance Repair: Once you've tackled the furnace, a toaster or blender seems like child's play. Find out how to fix them here.
    • Thermostat Maintenance: To make sure there's actually a problem with your heating system, you may want to check the thermostat, too. Learn how to calibrate a thermostat.
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