Steps to Fix Gas Furnaces

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Compared to oil burners, natural gas and propane burners are easier to operate, but they may also have some issues. Most problems that arise with gas furnaces involve the thermocouple, pilot light, or electrical components.

Gas furnaces and heaters have control shutoffs to prevent gas leaks, but they are not completely reliable. If you smell gas in your home, do not turn on or off any lights and avoid shutting off the gas leading to the furnace. Leave the house immediately, leaving the door open, and call the gas company or fire department to report the leak. Do not return to your home.

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Most natural gas furnaces have few operational difficulties. Problems typically
involve the pilot light, the thermocouple, or some part of the electrical system.

Some gas furnaces and heaters have a plug-type door that covers the pilot light assembly. To access the pilot burner, pull the door out of the furnace housing. In other units, remove the panel that covers the pilot and gas burners.

The pilot light controls, reset buttons, gas valves, and thermocouple are typically located in an assembly at the front of the furnace. The furnace limit switch is usually found on the plenum or main duct junction on the upper housing of the furnace.

Before attempting to fix your furnace, identify the problem to save time. Check the chart on the next page for common problems and solutions.

For more home repair articles, visit the links below.

  • Furnace Maintenance: Learn how to keep your furnace in excellent condition and save money and time.
  • Major Appliance Repair: If your furnace isn’t the only broken appliance in your home, learn how to fix other machines.
  • Small Appliance Repair: Once you’ve fixed the furnace, smaller appliances like toasters and blenders will be a breeze. Learn how to fix them here.
  • Thermostat Maintenance: Make sure your heating system has an actual issue by checking the thermostat. Learn how to calibrate a thermostat.

secure access panels.

2. Blower belt slipping
or broken.

2. Adjust or replace belt.

3. Furnace needs

3. Lubricate all moving parts
(if applicable).

Gas Furnaces and Gas Heaters Troubleshooting Tips

If your gas furnace or gas heater isn’t working properly, there are several potential reasons why. However, many solutions are simple and can be resolved with a few easy steps. This chart provides a range of solutions for different conditions that may prevent your gas furnace from working.

The chart includes common problems, possible causes, and suggested solutions. For example, if your furnace won’t run, it may be due to a lack of power, a switched-off power switch, a motor overload, a pilot light that has gone out, or a lack of gas. The chart provides step-by-step instructions for what to do in each of these cases.

If your furnace isn’t providing enough heat, it may be due to a thermostat that is set too low, a dirty filter, a clogged blower, closed or blocked registers, an imbalanced system, a loose or broken blower belt, or a dirty burner. The chart provides solutions for each of these issues.

If your pilot won’t light, it may be due to a blocked pilot opening or a lack of gas. If your pilot won’t stay lit, it may be due to a loose or faulty thermocouple, a pilot flame that is set too low, or an electric pilot that is faulty. The chart provides steps for addressing each of these issues.

If your furnace turns on and off repeatedly, it may be due to a dirty filter or a motor and/or blower that needs lubrication. If your blower won’t stop running, it may be due to a blower control that is set wrong or a limit switch that is set wrong or needs adjustment. If your furnace is noisy, it may be due to loose access panels, a slipping or broken blower belt, or a lack of lubrication in moving parts.

Using this chart, you can troubleshoot many common issues with your gas furnace or gas heater and get your system back up and running quickly and easily.

Make sure access panels are securely fastened. If the belts are sticking, worn or damaged, spray them with belt dressing or replace them. Adjust the blower belts if they are too loose or tight. If the motor and blower have oil ports, lubricate them. If the burner is dirty, call a professional. The pilot light is essential for the furnace’s operation. To learn how to light a pilot light, refer to the next page. Check out the following links for more home repair articles: Furnace Maintenance, Major Appliance Repair, Small Appliance Repair, and Thermostat Maintenance.

How To Light a Furnace Pilot Light

To check the pilot light, you’ll need matches and fine wire. The pilot light on a gas furnace may go out due to drafts. Follow the manufacturer’s instructions carefully to relight it. If instructions are not provided, use this general procedure:

To relight a gas furnace’s pilot light, first locate the pilot light assembly, which typically has a gas valve with on, off, and pilot settings. Turn the valve to off and wait three minutes. Then switch the valve to pilot setting and hold a lighted match to the pilot opening while pushing the reset button on the pilot control panel. Keep the button depressed until the pilot flame burns brightly, and then set the valve to the on position. If the pilot flame won’t stay lit, the opening may be clogged, and you should turn the gas valve off and clean the opening with a piece of fine wire. If the pilot still won’t stay lit after several attempts, you may have a faulty thermocouple, and you should call a professional service person. If your furnace has an electric ignition system, and it malfunctions, call a professional service person. To replace a faulty thermocouple, first, tighten the thermocouple nut with a wrench, being careful not to apply too much pressure. If the pilot still won’t stay lit, replace the thermocouple with a new one of the same type. To replace a thermocouple, unscrew the copper lead and connection nut inside the threaded connection to the gas line, and unscrew the bracket nut that holds the tube in place under the mounting bracket at the thermocouple tube. Insert a new thermocouple into the hole in the bracket, with the steel tube up and copper lead down, and then screw the bracket nut over the tube. Push the connection nut to the threaded connection where the copper lead connects to the gas line, and tightly screw the nut into place, but do not over-tighten. Both nuts should be only a little tighter than if hand-tightened.

The thermocouple is positioned beside the pilot light and secured in place by a bracket. The steel tube faces upward while the copper lead faces downward. This safety control switch, known as the limit switch, is located beneath the plenum on the furnace. Its purpose is to shut off the burner if the plenum becomes too hot. Additionally, it will switch off the blower once the temperature drops to a specific level after the burner has been turned off. If the blower runs continuously, it may be due to the blower control on the thermostat being set to ON or the limit control switch requiring adjustment. Start by checking the thermostat and changing the blower control from ON to AUTO. If it is already on AUTO and the blower continues to run, then the limit switch needs adjusting.

To adjust the switch on your gas furnace, first remove the control’s cover. Inside, you will see a toothed dial with two sides – LIMIT and FAN. The LIMIT side should not be touched. On the FAN side, there are two pointers indicating the upper and lower settings for the blower. Set the upper pointer to around 115 degrees Fahrenheit and the lower pointer to around 90 degrees Fahrenheit, with a 25 degree gap between them. If you smell gas during these adjustments, address any leaks immediately. To detect leaks, mix liquid detergent and water and paint it on the gas supply line, valves and connections, watching for bubbles. If you find a leak, tighten connections with a pipe wrench but be careful not to overtighten. For more information on home repair, visit our links for Furnace Maintenance, Major Appliance Repair, Small Appliance Repair and Thermostat Maintenance.


1. What are some common problems with gas furnaces?

Some common problems with gas furnaces include not producing enough heat, not producing any heat at all, making strange noises, having a faulty thermostat, and having a clogged air filter.

2. How often should gas furnaces be serviced?

Gas furnaces should ideally be serviced once a year to ensure their proper functioning. Regular maintenance can also prevent more serious problems from occurring.

3. Can I repair my gas furnace myself?

It is not recommended to attempt to repair a gas furnace yourself. Gas furnaces are complex machines that require specialized knowledge and training to fix. Attempting to repair it yourself could result in serious injury or even death.

4. How long do gas furnaces typically last?

Gas furnaces typically last between 15 and 20 years with proper maintenance. However, if a furnace has not been properly maintained, it may have a much shorter lifespan.

5. How can I tell if my gas furnace needs repair?

Some signs that your gas furnace may need repair include strange noises, insufficient heat, an increase in your energy bill, and a yellow or flickering flame.

6. How much does it cost to repair a gas furnace?

The cost to repair a gas furnace can vary greatly depending on the issue and the location. However, on average, a repair can cost anywhere from $100 to $1,000.

7. How can I maintain my gas furnace?

You can maintain your gas furnace by regularly changing the air filter, keeping the area around the furnace clean, and scheduling annual maintenance appointments with a professional technician.

8. What should I do if I smell gas coming from my furnace?

If you smell gas coming from your furnace, you should immediately evacuate your home and call the gas company or fire department. Do not attempt to turn off the gas or investigate the source of the leak yourself.

9. How can I prevent problems with my gas furnace?

You can prevent problems with your gas furnace by regularly maintaining it, keeping the area around it clean, and scheduling annual maintenance appointments with a professional technician.

10. How can I choose the right technician to repair my gas furnace?

You can choose the right technician to repair your gas furnace by researching and reading reviews from past customers, verifying their licensing and insurance, and asking for referrals from trusted friends and family members.

11. Can a gas furnace be repaired or does it need to be replaced?

Whether a gas furnace can be repaired or needs to be replaced depends on the specific problem and the age of the furnace. A professional technician can assess the situation and recommend the best course of action.

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