How to fix a furnace that smells like gas

If your furnace is giving off a gas smell, it’s important to address the issue right away. While it might seem alarming, many causes of this smell are fixable with a little DIY work. Of course, safety is always the top priority, so if you notice a strong gas odor, it’s essential to turn off the gas supply and avoid using the furnace until you can troubleshoot. We’ll walk you through the steps to figure out what’s causing the gas smell and how to fix it and keep your home warm and safe.

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 advice, it’s a good idea to check out your appliance’s manual or reach out to us if you have questions.

1. Turn off the furnace and gas supply

The first thing you want to do when you smell gas is shut everything down to avoid any potential dangers. It’s a quick and simple step that immediately keeps things safe while you troubleshoot. 

How to do it:

1. Locate the gas shut-off valve, which is usually right next to your furnace.
2. Turn the valve clockwise to completely shut off the gas.
3. Next, unplug the furnace or switch off the power at the circuit breaker. This will make sure the furnace isn’t running while you’re working on it.

2. Check for gas leaks

A strong gas smell usually means there’s a leak somewhere in the system. It’s important to identify and fix any leaks as soon as possible to prevent any safety hazards. Here’s how to check for gas leaks:

1. Use soap and water. Mix a small amount of dish soap with water in a spray bottle.
2. Spray the soapy water onto the gas connections, including the gas line, fittings, and valves.
3. Look for bubbles. If there’s a leak, you’ll see bubbles forming where the gas is escaping.
4. Be thorough and check all the connections and parts where gas could be leaking, including around the furnace’s gas valve.

If no leaks are found, proceed with the other troubleshooting steps.

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3. Inspect the flame sensor

A common cause of a gas smell can be a malfunctioning flame sensor. The flame sensor detects whether the burner has ignited, and if it’s dirty or not working correctly, it could cause improper gas flow or incomplete combustion, which can result in a gas odor. Here’s how to check and clean the flame sensor:

1. Locate the flame sensor, typically located near the burner assembly. Check your furnace manual for the exact location.
2. Before you begin, make sure to turn off the furnace and disconnect the power to avoid any accidents.
3. Unplug the flame sensor by unscrewing the mounting screws and disconnecting the wires.
4. Use a fine sandpaper or a non-abrasive pad to clean off any soot, dirt, or buildup. Gently scrub the sensor until it’s free of debris.
5. Use a multimeter to test the sensor’s continuity. Set the multimeter to measure resistance (ohms).
6. Attach the probes to the flame sensor terminals, and check for continuity. If the reading shows no continuity, the sensor may need to be replaced.
7. Once cleaned or replaced, reattach the sensor, secure it with screws, and reconnect the wires.

4. Inspect the burners

If your furnace burners are dirty or malfunctioning, it can cause inefficient combustion and lead to gas smells. A buildup of dirt, debris, or rust on the burners can affect the flame and result in an improper burn, which in turn creates the gas odor. Here’s how to inspect and clean the burners:

1. Always make sure the furnace is turned off and the gas supply is shut off before starting any repairs.
2. Open the furnace access panel and locate the burner assembly. This will typically be near the bottom of the furnace, connected to the gas valve.
3. Check the burners for rust, dirt, or any other debris that may have built up over time.
4. Look for any discoloration or signs of wear, which can indicate poor performance.
5. Use a soft brush or vacuum to remove any debris from the burner surfaces.
6. If necessary, use compressed air to blow out dust or soot from any smaller parts or parts you can’t reach easily.
7. If you see rust, clean the affected areas with a steel wool pad or brush.
8. Turn the furnace back on briefly to check the flame.
9. Check that the flame is steady and blue. If the flame is yellow or uneven, the burner might need to be replaced.

5. Check the gas valve and regulator

A faulty gas valve or regulator can cause improper gas flow, which might lead to a gas smell. If the gas valve isn’t working properly, it can result in excess gas leaking or not being distributed evenly, which may be why you’re noticing the odor. Here’s how to inspect and test the gas valve and regulator:

1. Start by locating the gas valve and regulator, which should be near the gas supply line and furnace.
2. Check for any visible signs of wear, corrosion, or physical damage to the gas valve and regulator.
3. Set your multimeter to measure continuity.
4. Disconnect the wiring from the gas valve.
5. Place the multimeter probes on the terminals of the gas valve and check for continuity. A working gas valve should show continuity. If there is no continuity, the gas valve may need to be replaced.
6. If the gas valve is working, but you’re still getting a gas smell, check the regulator.
7. Make sure the regulator is properly adjusted and functioning. If the gas pressure seems off or the regulator looks damaged, it may need replacing.
8. If you find either the gas valve or regulator is damaged or malfunctioning, it’s best to replace them.
9. To replace, disconnect the old parts carefully and install the new parts following the manufacturer’s instructions.
10. Tighten any connections securely to prevent leaks.

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6. Inspect the heat exchanger

A damaged or cracked heat exchanger can lead to dangerous situations, including gas leaks, which may cause a gas smell. The heat exchanger is responsible for transferring heat from the burner to the air circulated through your home. If it’s cracked, gas could leak into the furnace, resulting in a gas odor. Here’s how to inspect the heat exchanger:

1. Before inspecting the heat exchanger, make sure that the furnace is off and the gas supply is shut off for safety.
2. Open the furnace access panel and locate the heat exchanger. Depending on your furnace model, it might be in a different area, so check the manual for guidance.
3. Visually inspect the heat exchanger for any cracks, rust, or signs of wear. Pay special attention to areas where cracks may not be easily visible.
4. If you smell gas near the heat exchanger, or if there is soot or unusual discoloration around the area, these are signs of a potential crack or leak.
5. A flashlight can help you get a better view of the heat exchanger. Shine the light into the chamber and look closely for any cracks or holes.
6. If you’re not sure whether the heat exchanger is cracked, consider using a smoke test or a CO detector to check for gas leakage. You can also have the system pressure-tested if you suspect a serious problem.
7. If you find cracks or damage, it’s usually best to replace the heat exchanger. Consult your manual for instructions and follow the steps carefully to replace the heat exchanger.

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Additional information

For more tips on keeping your furnace and other appliances in top shape, swing by our repair help section. Knowing your appliance’s exact model is super helpful for troubleshooting. Find out how to locate your model number here.

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