⚠️ Difficulty: Medium — This repair involves working with high-voltage electrical components, gas lines, and fragile heating elements.
Prerequisites: You will need a multimeter that can measure DC microamperes (µA DC), a screwdriver set, and a nut driver set. Expect this job to take 1 to 2 hours. The process requires patience, careful disassembly, and strict adherence to safety protocols. If you do not feel confident working with gas or electricity, hire a professional.
Many homeowners believe that when a Lennox furnace refuses to ignite, the entire unit is shot and needs to be replaced. This is completely false. In reality, the problem is often something much simpler — and far cheaper — to resolve, such as a dirty flame sensor, a cracked igniter, or a blocked pressure switch tube. You do not need to replace the whole furnace.
A classic mistake people make when trying to fix a non-igniting furnace without proper instruction is using excessive force to open panels or touching the heating element of the Hot Surface Igniter (HSI) with bare hands. Skin oil residue creates hot spots on the igniter, causing it to crack prematurely. This guide will teach you the correct and safe method to diagnose and repair your Lennox furnace, step by step.
📝 Expert Note: This guide follows the manufacturer's recommended procedures for equipment maintenance, including the correct method for testing a Hot Surface Igniter using resistance measurements and verifying flame rectification current with a multimeter set to DC microamperes (µA DC).
How It Works
Your Lennox furnace uses a controlled sequence of events to ignite the gas-air mixture in the combustion chamber. When the thermostat calls for heat, the control board first activates the inducer motor, which creates a draft through the vent system. A pressure switch monitors this draft — typically requiring a negative pressure between -0.45 in. w.c. and -0.90 in. w.c. — to ensure proper venting before ignition is allowed.
If the pressure switch closes, the control board sends power to the Hot Surface Igniter (HSI). This igniter, made of either silicon carbide or silicon nitride, heats up to a very high temperature. Silicon carbide igniters operate at 120V and are quite fragile. Silicon nitride igniters are more durable and can operate at 80V or 120V. Once the igniter is glowing hot, the gas valve opens, and the gas-air mixture ignites.
A flame sensor then detects the presence of the flame using the principle of flame rectification. The flame ionizes the gases, allowing a small DC current — ideally between 2.0 and 5.0 µA DC — to flow through the sensor. If the control board does not detect this current, it shuts off the gas valve within seconds. Safety devices like the high limit switch (near the heat exchanger) and rollout switches (near the burners) prevent overheating and flame rollout.
Diagnosis Table: Physical Hazards During Repair
| Repair Phase | Physical Hazard | Required Protection / Action |
|---|---|---|
| Disconnecting Power & Gas | Electric shock; gas leak | Disconnect power at the service panel. Turn off gas supply valve. |
| Testing the Hot Surface Igniter | Burns from hot element; electric shock from 120V | Allow component to cool fully. Use insulated tools. Do not touch the element with bare hands. |
| Testing the Flame Sensor | Electric shock from control board terminals | Use multimeter probes carefully. Avoid shorting terminals. |
| Checking Pressure Switch | Diaphragm damage from excessive pressure | Do not blow or suck on the switch while connected. Human breath pressure can exceed 10 in. w.c. |
| Resetting Safety Switches | Overheating; fire; carbon monoxide leak | Never bypass limit or rollout switches for more than a few seconds of testing. Investigate root cause first. |
Price Table: Parts & Materials
| Material / Part | Estimated Price Range | Where to Find |
|---|---|---|
| Hot Surface Igniter (HSI) - Silicon Carbide | $15 – $40 | Online retailers, HVAC supply stores |
| Hot Surface Igniter (HSI) - Silicon Nitride | $25 – $60 | Online retailers, HVAC supply stores |
| Flame Sensor | $10 – $30 | Online retailers, HVAC supply stores |
| Pressure Switch | $20 – $50 | Online retailers, HVAC supply stores |
| Control Board | $80 – $200 | Online retailers, HVAC supply stores |
Tools & Materials List
- Multimeter capable of measuring AC/DC voltage, resistance (Ohms), and DC microamperes (µA DC)
- Screwdriver set (Phillips and flathead)
- Nut driver set
- Wire strippers/cutters
- Pliers
- Replacement Hot Surface Igniter (if needed)
- Replacement flame sensor (if needed)
- Replacement pressure switch (if needed)
- Non-metallic abrasive pad (like a Scotch-Brite pad) or ultra-fine steel wool (for cleaning the flame sensor)
Step-by-Step Repair
Now we enter the phase that separates the amateurs from the true DIY enthusiasts. It is time to calibrate your precision and follow each step methodically. Your furnace is a system of interdependent parts — understanding the 'why' behind each action will make you a better troubleshooter.
Step 1: Safety First — Disconnect Power and Gas
Before you touch anything, disconnect electrical power to the furnace at the service panel. Then, turn off the gas supply valve. This is non-negotiable. You are working with 120V AC and a flammable gas — a single mistake can be fatal.
Why this matters: The control board, igniter, and flame sensor all operate on live electrical circuits. Even with the power off, capacitors in the control board can hold a charge. Always verify with your multimeter that power is truly off before proceeding.
🔧 Expert Tip: Use a non-contact voltage tester to confirm the power is off at the furnace disconnect switch. This adds an extra layer of safety before you open any panels.
Step 2: Observe the Sequence of Operation
Turn the power back on temporarily and set the thermostat to call for heat. Watch the furnace from a safe distance. You should see the inducer motor start first. Then, listen for the pressure switch to click closed. Next, the Hot Surface Igniter should begin to glow. Finally, the gas valve should open with a soft "thump."
Why this matters: Each step in this sequence depends on the previous one. If the inducer motor runs but the igniter never glows, the problem is likely electrical — either the pressure switch didn't close, or the igniter itself has failed. If the igniter glows but no gas flows, the issue is with the gas valve or the control board signal.
⚠️ Note on Door Safety Switch: Most Lennox furnaces feature a blower door safety switch. When you remove the access panels to observe the components, this switch automatically pops out and cuts all power to the system. To observe the sequence of operation with the panels off, you must safely tape down or manually hold this switch. Remember to release it once your diagnostic testing is complete.
Step 3: Test the Hot Surface Igniter (HSI)
If the HSI does not glow, turn off the power and remove the igniter from its bracket. Use your multimeter set to resistance (Ohms) to test the two terminals of the igniter.
- For a silicon carbide igniter: The resistance should measure between 40 and 90 Ohms when cold.
- For a silicon nitride igniter: The resistance should measure between 11 and 17 Ohms when cold.
An infinite reading (OL) indicates an open circuit — the igniter is cracked and must be replaced. A reading outside these ranges suggests high resistance, which will prevent the igniter from heating properly.
Why this matters: The HSI is essentially a resistor that converts electrical energy into heat. If its resistance is too high or too low, it cannot reach the temperature needed to ignite the gas. Touching the element with bare hands creates oil residue that causes localized hot spots and cracking.
⚠️ Common Mistake: Many DIYers replace the igniter without testing it first. A glowing igniter does not always mean it is good — it may be glowing unevenly due to high resistance, which still prevents ignition. Always measure the resistance.
Step 4: Clean or Test the Flame Sensor
If the furnace ignites but then shuts off after a few seconds, the flame sensor is the most likely culprit. Locate the flame sensor — it is a metal rod positioned in the flame path near the burners. Remove it and gently clean the metal rod with a non-metallic abrasive pad (such as a red Scotch-Brite pad) or ultra-fine steel wool. Do not use standard sandpaper, as the silica residue left behind will melt under the flame’s extreme heat, vitrify on the metal surface, and permanently ruin the sensor. Reinstall it and test the furnace.
If cleaning does not resolve the issue, you need to test the flame rectification current. With the furnace running and the flame established, set your multimeter to DC microamperes (µA DC). Disconnect the flame sensor wire and connect your meter in series between the sensor terminal and the wire. The reading should be at least 0.5 to 1.0 µA DC, with an ideal range of 2.0 to 5.0 µA DC. If the reading is below the minimum threshold, replace the flame sensor.
Why this matters: The flame sensor relies on the flame's ionized gases to conduct a tiny DC current. Combustion byproducts coat the sensor rod, acting as an insulator and blocking this current. If the control board does not detect the rectification signal, it assumes the flame is not present and shuts off the gas valve for safety.
Step 5: Check the Pressure Switch
If the inducer motor runs but the pressure switch does not close, the furnace will not proceed to ignition. First, inspect the vent system for any blockages — leaves, bird nests, or ice can obstruct the flue. Next, examine the rubber tubing connecting the pressure switch to the inducer motor housing. Look for cracks, kinks, or blockages. If the tubing is clear and the vent is unobstructed, the pressure switch itself may have a failed diaphragm or electrical contacts.
Why this matters: The pressure switch is a safety device that ensures the inducer motor is creating adequate draft to expel combustion gases. If the switch does not close, the control board will not send power to the igniter. Never blow or suck on the pressure switch while it is connected to the control board — human breath pressure can exceed 10 in. w.c., which is far above the switch's operating range of -0.45 to -0.90 in. w.c., and can damage the internal diaphragm.
Step 6: Investigate Tripped Safety Switches
If the furnace has recently overheated or you smell gas, the high limit switch or rollout switches may have tripped. These switches are normally closed and open when they detect unsafe conditions. Before resetting any switch, you must investigate the root cause.
- High limit switch tripped: Check for a dirty air filter, restricted return air ducts, or a faulty blower motor. These cause the heat exchanger to overheat.
- Rollout switch tripped: This indicates flame is escaping the combustion chamber. Possible causes include a cracked heat exchanger or a blocked flue. Do not simply reset and ignore — this is a serious safety hazard that can lead to carbon monoxide leaks.
Why this matters: Safety switches are your furnace's last line of defense. Bypassing them for more than a few seconds of testing can lead to overheating, fire, or carbon monoxide poisoning. Always fix the underlying issue.
Step 7: Test the Control Board (Last Resort)
If all other components — igniter, flame sensor, pressure switch, and safety switches — test as functional, the control board may be faulty. Control boards can fail due to power surges, component degradation, or manufacturing defects. Replacing a control board is the most expensive and complex repair. Before ordering a new board, double-check every connection and verify that 120V power is reaching the furnace.
Why this matters: The control board orchestrates the entire ignition sequence. It sends timed signals to the inducer motor, igniter, and gas valve, while monitoring feedback from the flame sensor and pressure switch. A faulty board may fail to send power to the igniter or may not open the gas valve, even if all other components are working perfectly.
🛠️ Time to Call a Professional?
Ask yourself these questions:
- Do you have a multimeter that can measure DC microamperes (µA DC)?
- Have you worked with gas appliances and high-voltage electrical systems before?
- Are you comfortable disassembling the furnace to access the heat exchanger or control board?
If you answered "no" to any of these, calling a professional is the right choice — no shame. The cost of a service call is far less than the risk of fire, gas leak, or personal injury.
Curiosity & Data
Did you know that the Hot Surface Igniter technology was adapted from industrial furnace igniters used in steel manufacturing? In the early days of home heating, furnaces used a standing pilot light that burned gas continuously — a wasteful and dangerous system. The invention of the silicon carbide igniter in the mid-20th century allowed for intermittent ignition, saving energy and reducing the risk of gas leaks.
Silicon nitride igniters, introduced later, were a significant improvement. Silicon nitride is a ceramic material that is much tougher than silicon carbide. It can withstand thermal shock better and is less likely to crack from vibration or handling. This is why modern furnaces often use silicon nitride igniters, even though they are slightly more expensive.
The flame rectification principle used in flame sensors is a clever application of physics. When a flame burns, it ionizes the gases in the combustion zone, creating a conductive path. The flame sensor applies a small AC voltage, but because the flame acts as a diode — allowing current to flow more easily in one direction — the control board detects a DC current. This tiny signal, measured in microamperes, is enough to confirm that the flame is present and stable.
⚠️ Disclaimer: The information provided in this article is for reference and educational purposes only. The user assumes all responsibility for any repairs performed. Working with gas and electrical systems involves inherent risks. If you are uncertain about any step, or if local codes require licensed professionals, please hire a qualified HVAC technician.
Conclusion: Validation Protocol
You have done the hard work. Now it is time to validate your repair. Follow this checklist to ensure the problem is 100% resolved:
- Reconnect the gas supply and turn on the electrical power at the service panel.
- Set the thermostat to call for heat and observe the entire ignition sequence: inducer motor starts, pressure switch clicks, igniter glows, gas valve opens, and the burner ignites.
- Watch the flame for at least three full cycles. The flame should be steady and blue. If it flickers or goes out, the flame sensor may still be dirty or the gas pressure may be incorrect.
- After the furnace reaches the set temperature, listen for the blower to shut off and the burner to extinguish cleanly.
- Check for any gas odor near the furnace. If you smell gas, immediately turn off the gas supply and call a professional.
- Monitor the furnace for the next 24 hours. If the system cycles on and off without issues, your repair is successful.
If you managed to fix it, congratulations — you have saved yourself a service call and gained valuable knowledge about your home's heating system. If you chose to call a professional, that is also the correct decision. The important thing is that your furnace is safe and your home is warm.
FAQ
What is a Hot Surface Igniter and how does it work?
A Hot Surface Igniter (HSI) is a heating element made of silicon carbide or silicon nitride. When the control board sends power to it, the igniter heats up to a very high temperature, which then ignites the gas-air mixture in the combustion chamber.
How much does it cost to replace a flame sensor on a Lennox furnace?
A replacement flame sensor typically costs between $10 and $30. You can find them at online retailers or HVAC supply stores. The repair itself is straightforward if you have a screwdriver and a multimeter.
Can I clean a flame sensor instead of replacing it?
Yes, you can often clean a flame sensor with fine-grit sandpaper or an emery cloth. If cleaning does not resolve the issue, test the flame rectification current with a multimeter set to DC microamperes. If the reading is below 0.5 µA DC, replace the sensor.
When should I replace the pressure switch on my furnace?
Replace the pressure switch if the inducer motor runs but the switch does not close, and you have verified that the vent system and tubing are clear of blockages. A faulty diaphragm or corroded electrical contacts are common failure modes.
How can I prevent my furnace igniter from cracking?
Never touch the black or gray heating element with your bare hands. Skin oils create hot spots that cause the igniter to crack under thermal stress. Always handle the igniter by its ceramic base or bracket.
What does it mean if my furnace ignites but then shuts off after a few seconds?
This is a classic symptom of a dirty or faulty flame sensor. The flame sensor is not detecting the flame, so the control board shuts off the gas valve for safety. Clean the sensor first; if that does not work, test the flame rectification current.
How do I test a control board on a Lennox furnace?
Testing a control board is a last resort after verifying that the igniter, flame sensor, pressure switch, and safety switches are all functional. Check for 120V power at the board's input terminals and ensure all connectors are secure. If the board is not sending power to the igniter or gas valve, it may need to be replaced.
