How To Test Furnace Ignitor: A Professional Diagnostic Guide

Testing a furnace ignitor requires a digital multimeter set to measure ohms to check for electrical continuity or resistance within the manufacturer-specified range. A functioning silicon carbide ignitor typically reads between 40 to 90 ohms at room temperature, while a silicon nitride model reads between 10 to 70 ohms. If the multimeter displays infinite resistance or an open circuit, the ignitor has failed and must be replaced.

Pre-Operation & Equipment Checklist

Proper preparation ensures both personal safety and accurate electrical diagnostics when working with residential heating equipment. Furnaces combine high-voltage electricity and flammable gas, meaning safety precautions must take absolute precedence before opening the utility access panels.

• Essential Gear, Tools, and Materials:
  
  
  • Digital Multimeter (capable of measuring ohms and AC voltage)
  • Needle-nose pliers or a 1/4-inch nut driver (for removing access panels and wiring harnesses)
  • Flashlight or headlamp
  • Replacement furnace ignitor (matched to the exact OEM specifications if failure is confirmed)
  • Nitrile or cotton gloves (to protect fragile ignitor surfaces from skin oils)
• Mandatory Prerequisite Knowledge and Standards:
  
  
  • Familiarity with basic electrical safety protocols and digital multimeter operation.
  • Understanding of National Fuel Gas Code (NFGC) standards regarding appliance clearance and servicing.
  • Awareness that silicon carbide ignitors are extremely brittle and sensitive to thermal shock and oil contamination.
• Estimated Budget and Duration Benchmarks:
  
  
  • Time required: 20 to 45 minutes for a complete diagnostic procedure.
  • Cost of diagnostic tools: Zero, if a multimeter is already owned; professional replacement ignitors generally cost between 30 and 150 dollars depending on the HVAC make and model.

Step-by-Step Furnace Ignitor Diagnostic Workflow

Step 1: Disconnect Power and Gas Supply

Locate the main power switch for the HVAC system, typically mounted on the side of the furnace chassis or on an adjacent wall, and flip it to the off position. For added safety, turn off the breaker supplying power to the furnace in your main electrical service panel. Finally, close the manual gas shutoff valve located along the black iron gas supply line leading into the furnace gas valve to prevent any accidental fuel release during service.

Warning: Never bypass safety switches or attempt to test component voltage while live unless directed by advanced diagnostics, as line voltage (120V AC) inside a furnace can cause severe injury or death.

Step 2: Remove the Furnace Access Panel and Locate the Ignitor

Unscrew or unlatch the upper and lower furnace service doors to expose the internal burner compartment. Locate the burner assembly where the gas manifold meets the combustion chamber. The ignitor is positioned directly in front of one of the main burner ports, secured by a single mounting screw, and connected via a polarized plastic wiring harness.

Pro-Tip: Take a digital photograph of the wiring harness routing and component orientation before disassembly to ensure seamless reinstallation.

Step 3: Disconnect the Ignitor Wiring Harness

Trace the wires from the ceramic base of the ignitor back to the Molex or spade-terminal plug connecting it to the integrated furnace control board or the gas valve wiring loom. Firmly grasp the plastic housing of the connector and pull apart to disconnect the ignitor completely from the electrical circuit. Never pull directly on the delicate lead wires, as this can fracture the internal connections or detach them from the ceramic block.

Step 4: Inspect the Ignitor Element Visually

Carefully examine the glowing element portion of the ignitor, which is usually made of a grey, chalky ceramic material (silicon carbide) or a dark, metallic-looking ceramic (silicon nitride). Look for hairline cracks, blistering, structural sagging, or a white powdery ash buildup, all of which indicate component degradation. If the element is visibly cracked or broken in half, the ignitor is defective, and electrical testing is merely a formality.

Step 5: Perform the Multimeter Resistance Test

Set your digital multimeter dial to the ohms symbol (Omega) or the lowest resistance scale setting. Touch one multimeter probe to each of the metal terminals inside the ignitor plug. Observe the digital display to record the exact electrical resistance value across the circuit.

Furnace Ignitor Specifications and Material Properties

Common Site Failures and Field Fixes

• Symptom: Multimeter displays "OL" (Open Line) or infinite ohms.
  
  
  • Root Cause: The internal resistive matrix of the ignitor has burned through, creating a complete break in the electrical circuit.
  • Actionable Fix: Replace the faulty ignitor with an exact OEM-equivalent part, ensuring the replacement matches both voltage ratings and mounting dimensions.
• Symptom: Ignitor glows orange but fails to ignite the gas burners.
  
  
  • Root Cause: Low microamp flame rectification signal, weak gas valve output, or degraded ignitor surface area failing to reach the necessary ignition temperature (typically 1800 to 2500 degrees Fahrenheit).
  • Actionable Fix: Clean the flame sensor with fine emery cloth, check incoming gas pressure, and replace the ignitor if its thermal output is compromised by microscopic micro-fissures.
• Symptom: Resistance is within normal range, but the ignitor never receives power during a heating call.
  
  
  • Root Cause: A tripped rollout safety switch, a faulty draft inducer pressure switch, or a malfunctioning integrated furnace control board preventing the ignition sequence from starting.
  • Actionable Fix: Test upstream safety switches for continuity, clear any blocked exhaust venting, and inspect the control board fuses and relays.

Frequently Asked Questions

Can I touch a new furnace ignitor with my bare hands?

No, you should never touch the grey element of a silicon carbide or silicon nitride ignitor with bare skin. The natural oils and salts from human fingers leave a residue that creates a localized hot spot when the ignitor heats up, leading to premature thermal cracking and failure. Always handle the component by its ceramic or metallic bracket, or wear clean cotton or nitrile gloves.

What happens if my furnace ignitor has the wrong resistance?

If the resistance is significantly lower than the manufacturer specification, it will draw excessive current and potentially blow the control board fuse or transformer. If the resistance is too high, the ignitor will not draw enough current to reach the high temperatures required to light natural gas or propane, resulting in a system lockout.

Why does my hot surface ignitor keep breaking prematurely?

Frequent ignitor failure is often caused by excessive mechanical vibration from a failing blower motor, high line voltage surges from the electrical grid, or a gas valve that delays opening and subjects the ignitor to prolonged thermal stress. Ensure the furnace cabinet is properly secured, check line voltage stability, and verify burner alignment.

Can I test a furnace ignitor while it is still plugged into the control board?

It is strongly recommended to disconnect the ignitor from the control board before testing resistance to prevent false readings caused by parallel circuits or residual board capacitance. Measuring resistance on a live circuit will also damage your digital multimeter.

Professional Heating Maintenance and Diagnostic Support

Accurate diagnosis of your furnace ignition system prevents unnecessary component replacement and ensures safe, reliable home heating throughout the winter season. If your heating system continues to experience ignition failure after verifying the ignitor, contact a licensed HVAC professional for comprehensive combustion analysis and safety testing.