How to troubleshoot a gas furnace that won’t start

Dealing with a gas furnace that won’t start can be pretty frustrating, especially when it’s cold outside. But before you call in the professionals, you might be able to fix the issue yourself. This guide will take you through some common causes for a furnace failure to start, along with DIY steps to troubleshoot and get your furnace up and running without the need for an expensive repair! 

Keep in mind: The details in this article are intended as general advice and may not directly correspond with your particular appliance model. Differences in model features and configurations could mean variations in troubleshooting methods, error messages, or required part replacements. For the best results, consult your appliance’s technical documentation or user manual for guidance.

Safety precautions

When working on any appliance, safety should always come first. Here are some tips to keep in mind:

• Always turn off and unplug your appliance, or switch off the circuit breaker, before starting any maintenance or replacement work. This helps prevent electric shock and keeps you safe.
• Use insulated work gloves to protect your hands from sharp metal edges, pinching, and debris.
• Don’t rush through the job. Taking your time helps avoid accidents and injuries.
• Make sure you’re working in a well-lit area so you can easily see and access the inner parts.
• Keep your workspace tidy and free of clutter. It’s also a good idea to keep kids and pets away from the area.
• Never work on electrical parts with wet hands. Ensure your work area is completely dry.
• Check your user manual for any specific installation or safety instructions related to your appliance or parts.
• Handle parts with care. Using excessive force can damage the appliance or lead to injury.
• Wear safety glasses when working with chemicals, dust, or when cleaning up debris to protect your eyes.
• If the appliance has been in use recently, allow it ample time to cool down before you start working on it.
• Take photos or write down notes of wiring terminals or connections before disconnecting anything. This will help you avoid issues during reassembly.
• Avoid touching any exposed wires or terminals. If you need to touch a wire, use a non-conductive tool or wear insulating gloves to stay safe.

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Check the thermostat settings

One of the simplest things to check when your furnace won’t start is the thermostat. It might seem obvious, but sometimes the thermostat is accidentally set incorrectly. Here’s how to make sure the thermostat is working properly:

1. Check that the thermostat is set to the “heat” setting and not “cool.” If it’s on the wrong setting, the furnace won’t turn on.
2. Set the thermostat to a temperature higher than the current room temperature. This will signal the furnace to turn on and start the heating cycle.
3. If the thermostat display is blank or unresponsive, it could be faulty and you may need to replace the thermostat.

Verify the power supply

Without power, your furnace won’t be able to function, even if everything else is working. Here’s how to verify that the furnace is receiving the proper power:

1. Most furnaces have a dedicated power switch located near the furnace unit itself. Make sure this switch is turned to the “on” position.
2. Check the breaker that controls the furnace at your home’s electrical panel. If it’s in the “off” position or tripped, flip it back to “on.”
3. If the breaker keeps tripping, there might be an electrical issue or weak breaker that needs to be repaired.

Read more: 12 simple space heater safety tips for this winter

Check the pilot light (for older models)

If you have an older gas furnace with a pilot light, the furnace won’t start if the pilot light is out. A malfunctioning pilot light can prevent the furnace from igniting, so it’s a good idea to check it if your furnace isn’t starting. Here’s how to check and relight the pilot light:

1. Find the pilot light assembly on your furnace. It’s usually near the bottom of the unit, next to the burner assembly. There will be a small window or a glass sight where you can see the pilot light.
2. If the pilot is out, look for a button or knob on the furnace control panel labeled “pilot” or “reset.”
3. Turn the gas control knob to the “off” position and wait for at least five minutes to allow any residual gas to dissipate.
4. Turn the knob to the “pilot” position and hold it there while you press the ignition button or use a match to light the pilot light. Keep the knob pressed down for about 30 seconds to allow the thermocouple to heat up and maintain the flame.
5. Once the pilot light is lit, release the knob and turn it to the “on” position. The furnace should start running as soon as the pilot light stays lit.

If the pilot light won’t stay lit or keeps going out, the thermocouple might need to be replaced. The thermocouple senses the heat from the pilot light and sends a signal to the control board to keep the gas valve open.

Check for blockages in the vent system

1. Proper airflow is essential for furnace operation, and blockages can prevent your furnace from igniting or functioning efficiently. Here’s how to inspect and clear any potential blockages:
2. Check both the intake vent (where air enters the furnace) and the exhaust vent (where gases exit the furnace). Make sure these vents are not obstructed by debris, snow, leaves, or any other material.
3. Clear any visible obstructions from the vents. Ensure there’s nothing blocking airflow, as restricted airflow can cause the furnace to overheat and trigger safety switches to shut it off.
4. The flue pipe carries combustion gases outside the house. A blockage in the flue pipe can cause the furnace to shut down for safety reasons.
5. Look for any visible obstructions or damage to the pipe, such as bird nests or buildup. If you suspect a blockage, you can carefully remove it with a long brush or call for assistance to safely clear it.
6. If your furnace has a condensate drain (common in high-efficiency models), make sure it’s clear. A clogged drain can cause water to back up and trigger the safety shutdown.Check for any blockages in the drain line and clear them using a wet/dry vacuum or by flushing it with warm water.
7. Inside the furnace, there could be dust, dirt, or debris blocking airflow. If you feel comfortable, turn off the furnace and carefully inspect areas near the burner or heat exchanger for any buildup.
8. Clean any accessible areas with a soft brush or vacuum cleaner to remove dirt and dust.

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Looking for more?

When you need replacement parts, be sure to visit AppliancePartsPros.com with your model number. With millions of parts available, most ship fast and arrive in just two business days. For assistance, call our team at 1 (877) 477-7278.

While waiting for your part, check out our DIY blog and our YouTube channel, where you’ll find thousands of helpful repair videos.

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