How to fix gas furnace ignition problems

Gas furnace ignition issues are common problems that many homeowners face, and they can prevent your furnace from operating efficiently or at all. If your furnace isn’t igniting, running continuously, or making unusual noises, it could be an ignition problem. Ignition issues can stem from several parts of the furnace, including the ignitor, flame sensor, gas valve, thermostat, and electrical components.

Taking the DIY route to solve these problems can save you time and money. We’ll walk you through common ignition problems, how to identify them, and how to fix them yourself.

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   

Safety should always be a priority when working on any appliance. Here are a few practical tips to help you stay safe:

• Turn off the power to the appliance by unplugging it or switching off the circuit breaker before starting any maintenance or repairs. This protects you from the risk of electric shock.
• Wear insulated gloves to shield your hands from sharp edges, pinching hazards, or debris.
• Work at a steady pace and take your time to avoid accidents or injuries.
• Make sure your workspace is well-lit so you can clearly see and access the parts you’re working on.
• Keep the area clear of clutter, and ensure children and pets stay out of the workspace.
• Never handle internal parts with wet hands. Check that both your hands and the work area are completely dry.
• Refer to your appliance’s user manual for any specific instructions or safety guidelines related to the part or repair.
• Handle parts gently to avoid damage to the appliance or potential injuries from applying too much force.
• Use safety glasses when dealing with chemicals, dust, or large debris to protect your eyes from harm.
• If the appliance was recently used, give it enough time to cool down before working on any heated components.
• Take photos or notes of wiring connections and terminals before disconnecting anything to make reassembly easier.
• Avoid touching exposed wires or terminals. If you must handle wires, use non-conductive tools or wear insulated gloves to minimize the risk of electrical shock.

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Cause 1: Dirty or faulty ignitor

The ignitor is responsible for igniting the gas in the furnace. If it’s dirty or damaged, it can prevent the furnace from lighting, which will leave you with a cold house. Here’s how to identify and fix problems with the ignitor:

• Furnace fails to ignite or makes repeated clicking sounds without lighting.
• The furnace might try to start but then shut down after a short time.
• You might hear a humming or buzzing sound instead of the usual ignition noise.

How to clean or replace the ignitor:

1. Always start by turning off the furnace and disconnecting the power to avoid any electrical issues.
2. The ignitor is typically located near the burners or in the combustion chamber. Check your furnace’s manual if you’re unsure about its location.
3. Look for any visible damage or wear, such as cracks or discoloration.
4. If the ignitor looks dirty, use a soft cloth or fine wire brush to gently clean off any dust or residue.
5. If cleaning doesn’t resolve the issue or the ignitor is damaged, it’s time to replace it.
6. Take out the old ignitor by disconnecting the wires and unscrewing any fasteners.
7. Install the new ignitor in the same position, reconnect the wires, and secure it with screws.
8. Once the ignitor is cleaned or replaced, reassemble everything, reconnect the power, and test your furnace to see if it lights up.

Cause 2: Faulty flame sensor

The flame sensor is a safety feature in your furnace that detects whether or not the burner has lit. If the flame sensor is dirty or malfunctioning, it can cause the furnace to shut down prematurely, preventing it from reaching the desired temperature. Here’s how to identify a faulty flame sensor:

• Furnace starts, but then shuts off after a few minutes.
• The furnace runs but doesn’t heat properly.
• A flashing light or error code indicating a flame detection issue.

How to clean or replace the flame sensor:

1. For safety, always turn off your furnace and disconnect the power before performing any work.
2. The flame sensor is usually located near the burner assembly, with a metal rod sticking out into the flame.
3. Look for any signs of dirt, buildup, or corrosion on the sensor. A dirty sensor will prevent it from detecting the flame properly.
4. Use a soft cloth or fine steel wool to gently clean the sensor rod.
5. If cleaning doesn’t work or the sensor is damaged, it will need to be replaced.
6. Unscrew the mounting screws and disconnect the wires from the sensor.
7. Install the new flame sensor, ensuring it’s positioned correctly, and reconnect the wires.
8. Once the flame sensor is cleaned or replaced, put everything back together and test your furnace to see if it operates as expected.

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Cause 3: Clogged or dirty gas valve

A clogged or dirty gas valve can disrupt the flow of gas to the burner, which can prevent your furnace from igniting or cause it to run inefficiently. Here’s how to identify a problem with the gas valve:

• The furnace is not igniting or doesn’t run consistently.
• You hear clicking sounds but no ignition after several attempts.
• Gas smell near the furnace, indicating potential issues with gas flow.

How to clean or replace the gas valve:

1. Before working on the gas valve, make sure to turn off both the furnace and the gas supply to avoid any safety hazards.
2. The gas valve is typically located near the burner assembly. Check your furnace’s manual for the exact location.
3. Look for any visible dirt, debris, or blockages in the valve or its connections.
4. If the valve is clogged, clean it gently with compressed air or a soft cloth.
5. To ensure the valve is functioning correctly, use a multimeter to check if it’s allowing gas to flow properly.
6. Check for continuity across the valve’s terminals. If there’s no continuity, the valve may need to be replaced.
7. If the valve is beyond cleaning or if it’s damaged, replace it with a new one that matches your furnace model.
8. To replace the valve, disconnect the gas line, unscrew the old valve, and install the new one. Reconnect the gas line and ensure everything is tightened properly.
9. Once the gas valve is cleaned or replaced, reconnect everything, turn on the gas supply, and test the furnace to ensure proper operation.

Pro tip: If you smell gas at any point during this process, stop immediately, leave the area, and contact your gas supplier for assistance.

Cause 4: Thermostat issues

Your thermostat plays a crucial role in controlling the furnace’s operation, and if it’s not working properly, it can lead to furnace ignition problems or inconsistent heating. Here’s how to identify a thermostat issue:

• The furnace doesn’t turn on or off according to the set temperature.
• The thermostat settings seem unresponsive, or the furnace continues running after reaching the set temperature.
• The thermostat screen is blank or flickering.

How to check and troubleshoot the thermostat:

1. Ensure that the thermostat is set to “Heat” and not to “Cool” or “Off.”
2. Double-check the temperature setting to ensure it’s higher than the current room temperature.
3. If your thermostat uses batteries, check if the batteries are dead or low. Replace them with fresh batteries.
4. For wired thermostats, check if the circuit breaker for the thermostat is on. If it’s off, flip it back on.
5. Inspect the thermostat wiring. Turn off the power to the furnace and remove the thermostat cover and check the wiring behind it. Look for any loose, frayed, or disconnected wires.
6. If any wires appear damaged, carefully reconnect them or replace the wires as necessary.
7. Test the thermostat with a multimeter by setting your multimeter to check for voltage.
8. Place the probes on the terminals of the thermostat to check for proper voltage when the thermostat is set to “Heat.”
9. If the multimeter reads zero voltage or irregular readings, the thermostat may need to be replaced.
10. If the thermostat is still working but isn’t accurate, you can try recalibrating it. Follow the manufacturer’s instructions to reset or calibrate the settings to ensure accurate temperature readings.
11. If your thermostat is programmable, make sure the schedule is set correctly and matches your preferences.

If none of the above steps work, you may need to replace the thermostat. Purchase a replacement that matches the specifications of your current model.

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Additional information

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