Why a Furnace Ignitor Is Not Glowing: Causes and Solutions

The furnace ignitor not glowing is a common heating problem that leaves many homeowners without heat. This article explains the most frequent causes, diagnostic steps, safety considerations, and practical fixes to restore ignition. It focuses on electric hot surface ignitors and electronic spark ignitors found in modern U.S. gas furnaces and provides actionable troubleshooting guidance for technicians and informed homeowners.

Symptom	Likely Cause	Quick Action
Ignitor Does Not Glow	Broken ignitor, no power, failed control board	Visual inspection, multimeter check, replace ignitor
Ignitor Glows Dimly	Low voltage, poor connection	Check wiring and voltage
Ignitor Glows But No Flame	Gas valve, pressure, or flame sensor issue	Check gas supply and valve, clean sensor

How Furnace Ignitors Work

Modern gas furnaces use two common ignition types: hot surface ignitors (HSI) and spark ignitors. A hot surface ignitor is a silicon carbide or silicon nitride element that heats to 1,200–1,800°F when energized and then ignites gas at the burners. Spark ignitors produce an electrical arc that lights the gas immediately. Both rely on proper electrical supply, control signals from the furnace control board, and intact wiring to function correctly.

Common Causes When The Furnace Ignitor Is Not Glowing

Failed Ignitor Element

The most frequent reason a furnace ignitor is not glowing is a cracked or burned-out element. Hot surface ignitors are fragile and can fracture from thermal shock or vibration. A visible crack or broken piece is a clear sign the part must be replaced. Always inspect the element visually before further testing.

Loss Of Power Or Faulty Wiring

If the ignitor does not receive power it will not glow. Loose connector clips, broken wires, or corroded terminals often interrupt the circuit. Control boards send voltage during the ignition sequence; if wiring between the board and ignitor is damaged, the element remains unenergized.

Defective Furnace Control Board Or Relay

A failed control board or relay can prevent the voltage pulse that powers the ignitor. Control board failures can be intermittent; symptoms include no attempt to ignite, repeated lockouts, or no response to the thermostat. Diagnosing the board requires testing for expected voltages during the start sequence.

Blown Fuse Or Tripped Safety Switch

Many furnaces include internal fuses or safety switches that cut power to ignition components under fault conditions. A blown 3A or 5A fuse on the control board, a tripped limit switch, or an open rollout switch can stop ignition. These devices often protect the unit from overheating or flame rollout.

Low Line Voltage

Hot surface ignitors require a stable 120 VAC or specified control voltage. Low household voltage or a weak transformer can cause the ignitor to glow dimly or not at all. Voltage below the required threshold reduces heating and can prevent ignition.

Faulty Flame Sensor Or Gas Valve

In systems where the ignitor glows but burners don’t light, the issue may be gas-related. A faulty gas valve, closed gas supply, or incorrect manifold pressure prevents flame. A dirty or malfunctioning flame sensor may cause repeated shutdowns, but it typically does not stop the ignitor from glowing initially.

Tools And Safety Checklist Before Troubleshooting

Before attempting any troubleshooting, ensure safety: turn off power to the furnace at the service switch and at the breaker, and shut off gas if performing gas-valve work. Recommended tools include a digital multimeter, non-contact voltage tester, insulated screwdrivers, needle-nose pliers, and appropriate replacement parts. Wearing safety glasses and gloves is advised.

Step-By-Step Diagnostic Procedure

1. Verify Thermostat And Power

Ensure the thermostat calls for heat and that the furnace receives power. Check the furnace service switch (looks like a light switch) and the breaker. Use a non-contact voltage tester at the service switch and the furnace junction box if necessary.

2. Visual Inspection

Open the furnace access panel and visually inspect the ignitor for cracks, chips, or discoloration. Examine wiring, connectors, and the harness for loose or burned contacts. Look for blown fuses on the control board and signs of water or corrosion.

3. Test For Ignitor Voltage

Reset power, set thermostat to call for heat, and observe the ignition sequence. Use a multimeter to measure voltage at the ignitor connector during the call for heat. Hot surface ignitors should receive line voltage (commonly ~120 VAC). If no voltage is present, proceed to control board tests.

4. Resistance Check For Hot Surface Ignitors

With power off, disconnect the ignitor and measure resistance across its terminals. Typical readings range from 40 to 120 ohms for many HSIs, but specifications vary by model. An open reading (OL) indicates a failed element. Compare to manufacturer specs for accurate diagnosis.

5. Check Control Board Outputs And Relays

Test the control board for a switched output to the ignitor during the ignition sequence. If the board fails to send voltage but has proper power input, the board or its relay may be defective. Inspect relays for visible damage and test continuity when energized.

6. Inspect Safety Switches And Transformers

Test the transformer output and any limit or rollout switches in series with the ignitor circuit. An open safety switch interrupts voltage. Replace or repair faulty switches and verify transformer secondary voltage is within spec.

Common Fixes And Replacement Guidance

Replacing A Hot Surface Ignitor

Replacing an HSI is often the fastest fix. Order the correct OEM or compatible ignitor using the furnace model number. Handle the new ignitor by its ceramic body or mounting bracket; avoid touching the element surface with bare hands. Secure the replacement, reconnect wiring, and test the ignition sequence.

Repairing Wiring And Connectors

Tighten or replace corroded connectors and damaged wiring. Use heat-resistant high-temperature connectors where the harness passes near the burners or heat exchanger. Ensure proper routing to avoid vibration or chafing that could cause future failure.

Replacing Control Boards And Relays

Control board replacement is more complex and may require programming or dip-switch settings. Confirm the board is incompatible before replacing. If the board fails to send voltage despite receiving power and correct inputs, replacing it with an exact-match part is usually necessary.

Addressing Gas-Related Issues

If the ignitor glows but burners do not light, verify gas supply, the manual gas valve position, and line pressure. A stuck gas valve or clogged orifice may require a licensed HVAC technician or gas service professional. Do not attempt complex gas repairs without proper training.

When To Call A Professional

Call a licensed HVAC technician when the problem involves gas valves, manifold pressure, control board replacement, repeated ignition lockouts, or if the homeowner is uncomfortable with electrical testing. Professionals have the tools and safety training to diagnose control circuitry and handle gas components safely.

Preventive Maintenance To Avoid Future Ignitor Failures

• Keep the furnace compartment clean and dry; debris and moisture shorten ignitor life.
• Schedule annual HVAC tune-ups to inspect ignition systems and safety controls.
• Replace air filters regularly to prevent overheating that can stress internal components.
• Ensure proper burner alignment and periodic cleaning to reduce thermal cycling stress on the ignitor.

Costs And Typical Replacement Times

Ignitor parts are relatively inexpensive; hot surface ignitors typically cost between $30 and $120 for parts depending on brand. Labor for a licensed technician generally ranges from $75 to $200. Control board replacements cost more—parts and labor together can run several hundred dollars. Replacing an ignitor is often a 30–60 minute job for a technician.

Troubleshooting Checklist For Technicians

1. Confirm thermostat call for heat and power at furnace.
2. Visually inspect the ignitor element and wiring harness.
3. Measure ignitor resistance with power off.
4. Measure voltage to ignitor during ignition attempt.
5. Test control board outputs and relays for proper operation.
6. Verify fuses, limit switches, and transformer voltages.
7. Check gas supply and valve operation if the ignitor glows but burners fail to light.

Safety Tips And Best Practices

Always disconnect electrical power before working on the furnace. Use insulated tools when measuring live voltages and observe lockout/tagout procedures when needed. If a gas leak or smell is present, evacuate the area, shut off the gas at the meter if trained, and call the gas utility or emergency services.

Key Takeaways

The furnace ignitor not glowing usually indicates a failed ignitor element, loss of power, a faulty control board, or a tripped safety device. Effective troubleshooting follows a logical sequence: visual inspection, resistance and voltage testing, control board checks, and gas system verification. Minor fixes like replacing an ignitor or tightening a connector are common, but gas and control-board issues require professional service.

For safety and reliable repair, homeowners should weigh DIY tasks against calling a licensed HVAC technician when gas systems, control boards, or complex electrical diagnostics are involved.