How To Test A Furnace Ignitor: A Step-by-Step Multimeter Diagnostics Guide

To test a furnace ignitor, disconnect the electrical power to the furnace, locate the ignitor, unplug its wiring harness, and use a digital multimeter set to the lowest resistance setting (ohms). A functioning 120V silicon carbide ignitor should register between 30 and 150 ohms, while a silicon nitride ignitor typically reads between 40 and 100 ohms. An infinite resistance reading (OL or open loop) indicates a broken internal filament, meaning the ignitor has failed and must be replaced immediately.

Preparing the Diagnostic Workspace and Gathering Essential Tools

Testing a furnace hot surface ignitor (HSI) is a straightforward process, but it requires systematic preparation to prevent electric shock, system damage, or diagnostic errors. Before working on a gas furnace, you must establish a secure workspace, understand the basic electrical pathways, and gather precise diagnostic instruments.

Ignitors are highly fragile electrical components that operate at extremely high temperatures to ignite the gas flowing through the burners. If your furnace's inducer motor turns on and the blower runs, but you do not see a bright orange glow near the burners followed by combustion, a failed ignitor is the most probable cause.

Diagnostic Checklist and Safety Parameters

To ensure a safe and successful diagnostic procedure, review the following essential tools, required technical knowledge, and operational benchmarks.

• Essential Diagnostic Tools:
  
  
  • Digital multimeter (DMM) with resistance (ohms) and AC voltage measuring capabilities.
  • Insulated-handle nut drivers (typically 1/4-inch and 5/16-inch) or a magnetic screwdriver set.
  • Clean, nitrile gloves (crucial for handling new or existing ignitors to prevent skin oil contamination).
  • A bright, hands-free headlamp or tactical flashlight.
• Prerequisite Knowledge & Safety Standards:
  
  
  • Ability to locate the primary electrical service switch (furnace switch) and the corresponding circuit breaker at the main panel.
  • Understanding of basic multimeter operation, specifically how to set the dial to Ohms (Ω) and AC Voltage (V~).
  • Familiarity with the distinction between Silicon Carbide (older, highly fragile, flatter shape) and Silicon Nitride (newer, more durable, cylindrical/wrapped shape) ignitors.
• Time and Financial Benchmarks:
  
  
  • Estimated Duration: 15 to 30 minutes.
  • Estimated DIY Cost: $0 (assuming you own a basic multimeter).
  • Professional Diagnosis Cost: $100 to $250 (trip charge and diagnostic fee).
  • Replacement Part Cost: $15 to $80 depending on OEM (Original Equipment Manufacturer) specifications.

Detailed Electrical and Visual Testing Procedures

Follow this systematic, step-by-step procedure to isolate, inspect, and test your furnace ignitor. Do not skip any safety verification steps.

Step 1: Isolate the Electrical and Gas Power Systems

Before removing any access panel, turn off the electrical power to your furnace. Locate the toggle switch mounted on the side of the furnace casing or on a nearby wall (often looking like a standard light switch). Flip this switch to the "Off" position. To guarantee absolute safety, navigate to your home's main electrical breaker panel, locate the breaker labeled "Furnace" or "Heating," and switch it to the "Off" position.

Additionally, locate the manual gas shut-off valve on the incoming yellow or black iron gas pipe leading to the furnace. Turn the handle perpendicular to the pipe to shut off the gas supply.

Warning: Working on a live furnace can lead to severe electrical shock. Always use a non-contact voltage tester inside the furnace cabinet after removing the panel to verify that no live 120-volt current is present.

Step 2: Remove Cabinet Access Panels to Locate the Ignitor

Most modern furnaces feature two front doors: a top panel covering the burner assembly, control board, and gas valve, and a bottom panel enclosing the blower motor. Remove the upper service panel by lifting it upward or unscrewing the retaining fasteners.

Once the panel is removed, locate the burner assembly. The ignitor is positioned adjacent to the first burner slot, secured by a metal bracket and one or two screws. It features two wire leads that route back to a plastic quick-disconnect harness plug connected to the main control board wiring.

Step 3: Conduct a Micro-Visual Inspection

Carefully inspect the ceramic base and the gray heating element of the ignitor. Because silicon carbide ignitors are incredibly fragile, they frequently develop microscopic hairline fractures due to thermal expansion and contraction over years of service. Look for a tiny crack, often surrounded by a white, light gray, or powdery black discoloration spot. If you find any visible cracks or burned-out spots on the element, the ignitor is physically broken and will not function, regardless of what the electrical testing reveals.

Pro-Tip: Never touch the black or gray heating element of the ignitor with your bare fingers. Human skin leaves behind microscopic oil deposits. When the ignitor reaches its operating temperature of over 2,400 degrees Fahrenheit, these oils create a hot spot, causing the silicon to crack and fail prematurely.

Step 4: Disconnect the Wire Harness Connector

Trace the two wires running from the ceramic base of the ignitor to the plastic connector plug. Press down on the plastic locking tab of the male plug and gently pull it apart from the female receptacle.

Do not pull directly on the wires. Pull only on the plastic connector bodies to prevent tearing the copper wires from their terminals. Unplugging the harness is crucial because it isolates the ignitor from the rest of the furnace’s electrical circuits, preventing feedback loops that could cause inaccurate resistance readings.

Step 5: Configure Your Multimeter for Resistance Testing

Turn on your digital multimeter and rotate the dial to the resistance setting, indicated by the Ohm symbol (Ω). If your multimeter is manual-ranging rather than auto-ranging, set the scale to the lowest available setting, which is typically 200 ohms. If your meter has a continuity setting (which makes an audible beep when the leads touch), you can use it for a quick test, but a precise numerical resistance test is far superior for diagnosing weak or failing elements.

Step 6: Test Resistance (Ohms) at the Ignitor Plug Terminals

Insert your multimeter’s metal test probes directly into the two terminal openings on the isolated ignitor-side plug. Ensure that the metal probe tips make firm, clean contact with the metal crimps inside the plastic connector. Do not let your fingers touch the metal tips of the probes during this test, as your body's natural electrical resistance can alter the readings on the screen.

Read the value displayed on the digital screen:

• Silicon Carbide Ignitors: A healthy, standard silicon carbide element should register a resistance reading between 30 and 150 ohms.
• Silicon Nitride Ignitors: A durable silicon nitride element typically registers a lower resistance, generally between 40 and 100 ohms. Some specialized low-voltage OEM nitride ignitors may read as low as 11 to 17 ohms.
• Infinite/Open Loop (OL): If your meter reads "OL," "O.L," or "1" (on older manual meters), this indicates infinite resistance. The internal circuit of the element is broken. The ignitor must be replaced.
• Excessively High Resistance: If your carbide ignitor registers over 200 or 300 ohms, the material has degraded significantly. Even if it glows slightly, it cannot draw enough current to reach the temperature required to ignite the gas, and it will trigger a flame-rectification lockout.

Step 7: Verify Control Board Voltage (Advanced Step)

If your ignitor passed the resistance test but still does not glow during the startup sequence, the problem may lie in the furnace control board rather than the ignitor itself. To verify this, perform an AC voltage test. Set your multimeter to AC Volts (V~ or VAC) on a scale that can read up to 120 volts.

Insert your multimeter probes into the furnace-side female plug (the harness coming from the control board). Have an assistant restore power and call for heat by raising the thermostat setting. Watch the inducer motor start.

After the inducer runs for 15 to 45 seconds, the control board should send 120 volts AC to the plug. If your meter registers 110 to 125 volts for a brief window (usually 15 to 30 seconds before locking out), but the ignitor fails to heat up, the ignitor is faulty. If no voltage is sent to the plug during this sequence, the control board relay or integrated furnace controller is defective.

Furnace Ignitor Specifications and Diagnostic Thresholds

This comparative data reference highlights the physical properties, operational voltages, and diagnostic thresholds of the two primary categories of hot surface ignitors used in modern gas furnaces.

Troubleshooting Common Ignitor Failures and System Symptoms

The following real-world failure scenarios outline practical diagnostic pathways when troubleshooting ignitor problems in the field.

Scenario 1: The multimeter displays "OL" during the resistance test, and the furnace blower blows cold air.

• Root Cause: The silicon element has suffered a complete structural failure, breaking the internal electrical path. Because current cannot flow through the element, it cannot generate heat. The furnace's control board detects that ignition has not occurred via the flame sensor and shuts down the gas valve, leaving only the blower running to purge the heat exchanger.
• Actionable Fix: Replace the broken ignitor with an identical OEM or high-quality universal replacement part. Avoid touching the new element with bare hands.

Scenario 2: The ignitor resistance checks out perfectly, but it never glows, and there is no voltage at the wire harness during the startup sequence.

• Root Cause: The furnace's integrated control board is not sending power to the ignition circuit. This is caused by a failed onboard ignition relay, a tripped high-limit switch, or an open draft pressure switch that prevents the ignition sequence from advancing.
• Actionable Fix: Inspect the furnace's diagnostic LED indicator light for error codes (visible through the sight glass on the blower door). If the LED codes indicate an open limit or pressure switch, resolve that airflow restriction first. If the sequence is healthy but no power leaves the board to the ignitor, replace the control board.

Scenario 3: The new silicon carbide ignitor burned out and cracked after only a few weeks of service.

• Root Cause: The ignitor was likely contaminated with skin oils during installation, or the furnace control board is sending incorrect or high voltage. Alternatively, the bracket may be loose, causing severe mechanical vibration when the inducer motor or blower motor fires up.
• Actionable Fix: Install another replacement ignitor, taking care to wear clean nitrile gloves. Ensure that the mounting bracket screws are tightened securely to eliminate vibration. Verify with your multimeter that the incoming line voltage to the furnace is stable and does not exceed 125 volts AC.

Frequently Asked Questions

Can I test a furnace ignitor without a multimeter?

You can perform a visual test by observing the furnace during its startup sequence. If the inducer motor runs but you do not see a bright orange glow through the burner viewport before the gas valve clicks, the ignitor is likely bad. However, only a multimeter can confirm whether the failure is due to a broken ignitor element, a bad control board, or a tripped safety limit switch.

What should the resistance reading be on a good furnace ignitor?

A healthy silicon carbide ignitor should register a resistance reading between 30 and 150 ohms. A silicon nitride ignitor generally reads between 40 and 100 ohms. Any reading that shows "OL" or is significantly higher than 150 ohms indicates a faulty component that must be replaced.

Why does my furnace ignitor keep cracking and failing?

Premature cracking is usually caused by skin oil contamination from touching the gray element during installation, which creates destructive hot spots. It can also be caused by excessive furnace vibration due to an unbalanced blower wheel or a loose mounting bracket, as well as voltage surges from a faulty control board.

Is a furnace ignitor the same thing as a thermocouple?

No, they are entirely different components. A furnace ignitor is an electronic heating element that creates heat using electrical resistance to light the burners. A thermocouple is a safety device used in older furnaces with standing pilot lights that generates a tiny millivolt electrical current when heated by the pilot flame to keep the main gas valve open.

Restoring Home Heating with Expert Support

If you have completed these diagnostic tests and discovered a faulty ignitor, replacing the component will restore your heating system to optimal operation. For complex electrical readings, persistent failures, or professional furnace repair and replacement, contact a certified HVAC technician today.