How To Repair Gas Furnaces


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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