How To Tell If A Furnace Ignitor Is Bad: The Definitive Diagnostic Guide

A failing hot surface ignitor prevents your furnace burners from lighting, typically resulting in a system that blows cold air or locks out after a series of repeated glow attempts. By combining a visual inspection with a digital multimeter resistance test, homeowners and technicians can accurately determine whether the silicon nitride or silicon carbide ignitor has failed open.

Pre-Operation & Equipment Checklist

Diagnosing a faulty furnace ignitor requires systematic preparation, basic electrical safety protocols, and standard diagnostic instruments. Modern residential forced-air heating systems rely on electronic surface ignitors operating at 120 volts AC (or 24 volts AC in specific intermittent pilot setups) to ignite natural gas or propane.

• Essential Gear, Tools, and Materials:
  
  
  • Digital Multimeter (capable of measuring Ohms / resistance)
  • 1/4-inch nut driver or screwdriver set for accessing the burner compartment
  • Flashlight or headlamp
  • Clean cotton gloves or a microfiber cloth (skin oils damage new ignitor surfaces)
• Mandatory Prerequisite Knowledge and Standards:
  
  
  • Familiarity with basic electrical safety (NFPA 70 / NEC standards)
  • Understanding of standard HVAC sequence of operation: thermostat call, inducer motor start, pressure switch validation, ignitor warm-up, gas valve opening, flame sensing.
• Estimated Budget and Duration Benchmarks:
  
  
  • Diagnostic Time: 15 to 30 minutes
  • Replacement Cost: $20 to $60 for the part alone, or $150 to $350 for professional installation.

Step-by-Step Furnace Ignitor Diagnostic Workflow

Step 1: Secure Power and Remove Access Panels

Before touching any internal heating components, eliminate the risk of electrical shock and high-temperature burns. Locate the main service switch mounted on the side of the furnace chassis or turn off the dedicated breaker at your home electrical service panel.

Warning: Never test an ignitor circuit while the furnace is actively powered unless you are performing a live voltage drop test with insulated leads. Live 120V circuits present severe electrocution risks in cramped residential utility spaces.

Remove the upper blower/burner compartment door panel. Most modern furnaces feature a door safety interlock switch that cuts 120V power when the panel is removed, but shutting off the breaker or service switch remains the mandatory primary safety step. Locate the burner assembly where the ignitor sits directly in front of the burner ports, secured by a single mounting screw and connected via a white or clear plastic plug-in molex connector.

Step 2: Perform a Visual Inspection of the Heating Element

Examine the ignitor's fragile heating element—usually constructed of dark gray silicon carbide or lighter gray silicon nitride—without touching it directly. Look for structural fractures, hairline cracks, blistering, or a white powdery ash buildup.

Pro-Tip: If you notice a visible break, crack, or blister anywhere along the ignitor matrix, the component is completely dead. Silicon carbide and silicon nitride elements are brittle and cannot be repaired with adhesives or electrical tape; a cracked ignitor must be replaced immediately.

If the ignitor appears visually intact, look for discoloration. A mottled, dark spot right at the tip where the electrical arc or high resistance concentrates often indicates the material is thinning out and nearing total failure.

Step 3: Disconnect and Test Electrical Resistance with a Multimeter

Unplug the ignitor wire harness from the control board or main wiring plug. Set your digital multimeter to the lowest resistance setting (typically 200 ohms or the auto-ranging ohm symbol). Touch the multimeter probes to the two metal terminals inside the plastic connector plug.

Observe the digital readout on your screen. A healthy hot surface ignitor typically registers between 10 ohms and 170 ohms of resistance, depending on the specific voltage, manufacturer, and material composition (silicon nitride models often fall between 40 and 70 ohms cold, while older silicon carbide models can range up to several hundred ohms).

If your multimeter displays an "OL" (open line) or infinity symbol, the internal circuit of the ignitor is completely broken, confirming a dead component. If the resistance reading fluctuates wildly, the internal matrix is degrading and will fail under operating thermal loads.

Step 4: Verify Operational Voltage During a Heating Cycle

If the resistance test yields a normal reading, reconnect the ignitor and restore power to the furnace. Adjust your thermostat at least 5 degrees above room temperature to initiate a call for heat.

Watch the sequence through the viewing window or with the safety switch manually depressed (if local codes and safety practices permit). Listen for the inducer draft motor to start, followed by the pressure switch clicking closed. Watch the ignitor terminals or the element itself.

Using your multimeter set to AC Voltage (600V scale), carefully back-probe the ignitor harness plug while the control board attempts to light. You should read 120 volts AC (or 24V AC depending on system specs) supplied to the ignitor for a duration of 4 to 10 seconds. If power is reaching the plug, the ignitor glows brightly, but the burners fail to light, check for a dirty flame sensor, blocked gas supply, or faulty gas valve. If no voltage reaches the plug during the ignition cycle, the issue lies upstream at the integrated furnace control board.

Furnace Ignitor Technical Specifications Matrix

Common Site Failures and Field Fixes

• Root Cause: Oils from bare human fingers transferred to the ignitor surface during installation.
  
  
  • Actionable Fix: Always wear clean cotton gloves or handle new ignitors exclusively by their ceramic base. Wipe down accidental skin contact points with rubbing alcohol and let them dry completely before firing the system.
• Root Cause: High incoming line voltage from the utility transformer reducing ignitor lifespan.
  
  
  • Actionable Fix: Measure incoming AC voltage at the furnace transformer. If line voltage consistently exceeds 124V AC, consult an electrician or install a step-down transformer to protect sensitive electronic components.
• Root Cause: Excessive vibration from a failing inducer motor causing premature fracturing of the brittle ignitor element.
  
  
  • Actionable Fix: Inspect the inducer motor assembly for loose mounting bolts, worn bearings, or unbalanced blower wheels. Replace the inducer motor if excessive physical vibration persists.
• Root Cause: Corroded or loose molex connector terminals causing high electrical resistance and melting at the plug.
  
  
  • Actionable Fix: Inspect the wiring harness plug housing. If plastic discoloration or terminal melting is present, cut off the old connector and splice in a new manufacturer-approved ceramic wiring harness pigtail using high-temp ceramic wire nuts.

Frequently Asked Questions

Can a furnace ignitor glow orange and still be bad?

Yes. An ignitor can glow a dull orange or partial red yet fail to reach the high temperature (roughly 2400°F to 2800°F) required to ignite natural gas. If the internal resistance has drifted too high due to material aging, it lacks the electrical current draw needed to generate sufficient thermal energy, causing the safety control board to time out and shut down the gas valve.

How often should a residential furnace ignitor be replaced?

Most hot surface ignitors last between 3 to 7 years, depending on the frequency of heating cycles and the quality of the material. Silicon nitride models generally outlast older silicon carbide units. It is best treated as a wear-and-tear maintenance item rather than a permanent part.

Is it safe to touch a brand-new replacement ignitor with my bare hands?

No. Natural oils and salts from human skin leave a residue on the silicon surface. When the ignitor heats up to extreme temperatures, these oils create a localized hot spot that burns out the material prematurely. Always handle replacement ignitors by their ceramic brackets or wear clean gloves.

Why does my furnace keep clicking and trying to light without success?

This repetitive clicking typically indicates that the ignitor is glowing, but the gas valve is failing to open, or the flame sensor is not registering the resulting fire. If the ignitor glows for several seconds and then shuts off without gas ignition, check your gas supply shut-off valve, test the gas valve solenoid coil resistance, or clean the flame sensor rod with fine emery cloth.

Can I use a universal ignitor instead of an OEM part?

Yes, universal hot surface ignitors work effectively in most forced-air furnaces provided you match the operating voltage (120V vs 24V), physical mounting bracket configuration, and electrical plug style. Always verify that the electrical draw and physical dimensions match your specific furnace model specifications to prevent clearance issues with the burner tubes.

Professional Heating Diagnostics and Repairs

If your heating system continues to experience ignition lockouts or electrical faults after verifying the ignitor status, consult a licensed HVAC technician to inspect your integrated control board and gas manifold pressures. Ensure your home stays warm and efficient this winter by scheduling a comprehensive professional furnace tune-up today.