Diagnosing and Fixing a Bad Furnace Ignitor

The furnace ignitor is a small component with outsized impact: when it fails, heating stops, calls escalate, and energy waste can increase. This article explains how to identify a bad ignitor, the common causes, diagnostic steps, safe troubleshooting, replacement guidance, and when to call a professional, helping homeowners make informed decisions. Safety and correct diagnosis are emphasized throughout to avoid unnecessary repairs or hazards.

Symptom	Likely Cause	Recommended Action
Furnace won’t light	Cracked or failed ignitor	Visual inspection and multimeter test; replace ignitor
Intermittent heating	Weak ignitor or loose wiring	Check wiring and flame sensor; replace if resistance abnormal
Clicking but no ignition	Ignitor not reaching ignition temperature	Test electrical continuity and voltage; replace ignitor
Furnace trips breaker	Shorted ignitor or electrical fault	Turn off power; inspect for shorts; call technician if uncertain

What Is A Furnace Ignitor And Why It Matters

An ignitor is an electronic component that creates the heat or spark needed to ignite furnace gas. Modern furnaces typically use hot surface ignitors (HSI) made from silicon carbide or silicon nitride, which glow red-hot to light the gas. Other systems use spark ignitors or intermittent pilot systems. A functioning ignitor is essential for efficient, safe furnace operation; without it, burners cannot ignite and the furnace will lock out to prevent unsafe conditions.

Common Symptoms Of A Bad Ignitor

Recognizing ignitor failure early reduces downtime and prevents additional component stress. Common signs include: furnace won’t light despite thermostat calling for heat, continuous clicking or repeated ignition attempts, furnace starting then shutting down quickly, and visible cracks or dark spots on the ignitor element. Occasionally the furnace will run intermittently, indicating a marginal or failing ignitor.

Typical Causes Of Ignitor Failure

Ignitors fail for several reasons. Mechanical stress from cycling and vibration causes micro-cracks, while thermal stress from rapid heating and cooling degrades the element. Contamination from dust, oil, or residue can shorten lifespan, as can improper voltage or electrical surges. Age and manufacturer quality are factors; most hot surface ignitors last between 3,000 and 5,000 cycles or several years under normal conditions.

Safety First: Preparations Before Troubleshooting

Before any inspection or tests, the furnace must be powered down to avoid electric shock or burns. Turn off the thermostat, switch off the furnace power at the breaker and the service switch, and shut the gas supply if present. Use a flashlight and insulated tools. If there is any uncertainty about gas or electrical systems, contact a licensed HVAC technician. Never touch an ignitor while the furnace is powered or hot; the elements are fragile and can break from minor contact.

Visual Inspection Steps

Visual checks are the fastest initial diagnostic tool. Remove the furnace access panel and locate the ignitor near the burners or pilot assembly. Look for visible cracks, chips, or darkened spots on the ignitor surface. Inspect wiring and connectors for corrosion, loose connections, burn marks, or melted insulation. If the ignitor looks damaged, replacement is typically necessary rather than repair.

Using A Multimeter To Test The Ignitor

A multimeter provides a reliable way to test ignitor continuity and resistance. Set the meter to the ohms setting and measure across the ignitor terminals. For most hot surface ignitors, resistance ranges between 40 and 90 ohms, though values vary by model. A reading of infinite or open circuit indicates a broken element. A very low resistance could indicate a short. Refer to the furnace or ignitor specifications for exact values. Always test with power off and after the element has cooled.

Voltage And Control Board Diagnostics

If the ignitor tests okay but the furnace still won’t light, the issue may be in the control circuit. Verify that the control board is sending proper voltage to the ignitor during a call for heat. This typically requires measuring line voltage at the ignitor connector when the thermostat commands heat. No voltage often indicates a failed control board, faulty sequencer, limit switch, or safety interlock. These components should be diagnosed by a professional if the homeowner lacks the tools or experience.

Common Related Issues: Flame Sensor, Gas Valve, And Ignition Control

A bad ignitor can be confused with other ignition-related faults. A dirty or failing flame sensor will cause frequent lockouts even when ignition occurs. Gas valve problems or clogged burners can prevent ignition or produce weak flames. The ignition control module regulates timing and safety checks; failures here can prevent the ignitor from receiving power. Troubleshooting should follow a logical order: ignitor check, flame sensor cleaning, gas supply and burner inspection, then control circuitry.

Replacing A Furnace Ignitor: Step-By-Step Guide

Replacing an ignitor is a common homeowner or technician task when basic mechanical skills are available. Steps: power down furnace and shut gas; remove access panel; discharge any capacitors if present; locate and document ignitor wiring orientation; unplug connectors and remove mounting screws; install new ignitor without touching the element surface; reassemble, restore power, and test furnace operation. Use OEM or manufacturer-recommended ignitors to ensure correct resistance and fit. Handle the replacement part by its base to avoid oils contaminating the element.

Parts, Costs, And How To Choose The Right Ignitor

Ignitor costs vary widely. Parts for residential furnaces generally range from $30 to $150, depending on model and material. Silicon nitride ignitors cost more but are more durable than silicon carbide. When choosing a replacement, match part numbers or cross-reference specifications like voltage, resistance, and mounting style. Buying from reputable HVAC suppliers or the furnace manufacturer reduces compatibility issues. Consider warranty length and supplier return policies.

When To Call A Professional

Some situations call for a licensed HVAC technician: if gas system components require repair, if electrical diagnostics beyond simple continuity testing are required, if the control board may be faulty, or if the homeowner is uncomfortable with gas or electrical systems. Technicians provide safe diagnosis, can replace multiple components, and will confirm that replacement parts meet code and manufacturer requirements. For older furnaces close to end of life, a technician can advise whether repair or replacement is more cost-effective.

Preventive Maintenance To Extend Ignitor Life

Routine maintenance extends ignitor life and overall furnace reliability. Annual furnace tune-ups that include cleaning burners, replacing filters, inspecting wiring, and verifying ignition sequences reduce contamination and thermal stress. Minimizing rapid cycling by ensuring thermostat and system settings are appropriate helps too. Homeowners should replace air filters regularly, keep the furnace area clean, and schedule professional inspections before the heating season.

Troubleshooting Checklist For Homeowners

• Power Check: Ensure thermostat and breaker are on and furnace service switch is up.
• Visual Inspection: Look for cracks on the ignitor and damaged wires.
• Filter And Vents: Replace dirty filters and clear blocked vents that cause short cycling.
• Test Continuity: Use a multimeter to check resistance against manufacturer specs.
• Observe Ignition: Watch the ignition sequence: clicking, glowing, gas valve opening.
• Call Pros If: Gas smell, tripped breakers, or unfamiliar electrical testing needed.

Common Myths And Misconceptions

One myth is that an ignitor can be repaired by coating it or applying adhesives; this is false and unsafe. Another misconception is that frequent replacement indicates a defective furnace; while poor electrical supply or contaminants can shorten life, repeated failures often point to broader system or installation issues. Finally, some believe that any clicking means ignitor failure; clicking can be control board sequencing or gas valve attempts, so it requires proper diagnosis.

Energy And Safety Considerations

A failing ignitor can increase energy waste by causing repeated ignition attempts and cycling. Persistent ignition failures also stress the gas valve and control board, increasing the risk of component failure. While modern furnaces include safety lockouts to mitigate hazards, a professional inspection ensures the system operates within safety margins. Carbon monoxide risks are low for ignition-only failures but remain a concern for any incomplete combustion or venting issues.

Resources And Further Reading

For detailed specifications, homeowners should consult the furnace owner’s manual or the manufacturer’s website for part numbers and resistance values. Reputable HVAC forums and manufacturer support lines can help cross-reference parts. Licensed HVAC professionals provide diagnostics, permits, and safe repairs when gas or electrical systems are involved. Local utility or energy-efficiency programs may offer inspections or rebates for furnace upgrades.

Quick Reference: Typical Ignitor Resistance Values

Ignitor Type	Typical Resistance
Silicon Nitride Hot Surface Ignitor	40–60 Ohms
Silicon Carbide Hot Surface Ignitor	50–100 Ohms
Spark Ignitor	Not Applicable (High Voltage Spark)

Key Takeaways For Homeowners

Ignitors are fragile, vital components. Visual cracks, open-circuit readings, or inconsistent ignition generally mean replacement is necessary. Routine maintenance and correct part selection reduce future failures. For any gas or electrical uncertainty, a licensed HVAC technician should perform diagnosis and repair. Timely action avoids bigger repairs and keeps the home safe and comfortable.