Furnace Ignitor Not Getting Voltage: Causes, Diagnosis, and Fixes

The furnace ignitor not getting voltage is a common heating problem that prevents a furnace from lighting and delivering heat. This article explains likely causes, step-by-step diagnostic checks, safety precautions, repair options, and cost considerations so homeowners can troubleshoot effectively and decide when to call a professional. Key focus: identify why an ignitor isn’t receiving voltage and how to resolve it safely.

Symptom	Possible Cause	Quick Action
Ignitor Does Not Glow	No 120/24V supply, failed control board, open safety switches	Check power, fuses, transformer, safety switches
Clicking But No Ignition	Gas valve not getting 24V, wiring issue	Test for 24V at gas valve and ignitor circuit
Blower Runs But No Light	Flame rollout limit, pressure switch open	Inspect venting, check pressure switch and hose

How A Furnace Ignitor Works

A typical modern furnace uses a hot surface ignitor (HSI) or an intermittent pilot ignitor. The control board sends voltage to the ignitor during the heat call to create the spark or heat element that lights the burners. The ignitor circuit only receives voltage if upstream safety components—thermostat, transformer, fuses, limit switches, pressure switch, and control board—permit it.

Common Reasons A Furnace Ignitor Is Not Getting Voltage

Understanding root causes helps narrow diagnostic steps. Common reasons include: failed transformer, blown fuse on control board, bad thermostat wiring, open safety switches (limit, rollout, pressure), defective control board, loose or corroded wiring, and improper sequencing due to stuck gas valve or flame sensor issues.

Transformer Or Low Voltage Supply

The transformer steps down 120V household power to furnace control voltage (commonly 24VAC). A burned or shorted transformer will prevent voltage reaching the ignitor circuit, so testing the transformer’s secondary is an essential step.

Blown Control Board Fuse Or Tripped Fuse

Many furnace control boards have a 3A or similar mini fuse that protects the 24VAC circuitry. If this fuse is blown the ignitor will not receive voltage. Replacing the fuse may restore function, but identifying what caused the fuse to blow is required to prevent recurrence.

Faulty Control Board Or Ignitor Relay

The control board contains relays and solid-state switches that send voltage to the ignitor. A defective relay or damaged board traces can prevent that output. Diagnosing involves checking for the control board command signals and measuring voltage at the ignitor harness when a heat call is active.

Open Safety Switches (Limit, Rollout, Pressure)

Safety devices such as high-limit switches, rollout sensors, and pressure switches will interrupt the ignitor circuit when they detect unsafe conditions. An open pressure switch due to blocked venting or a clogged condensate drain will stop the furnace from energizing the ignitor.

Thermostat Or Wiring Issues

If the thermostat wiring is loose or misconfigured, the board may not receive a heat call. Verify R and W connections and confirm the thermostat battery and settings. Shorted or pinched wires can also interrupt the 24V control circuit.

Corroded Or Damaged Wiring And Connectors

Over time connectors and harnesses can corrode or become disconnected. Damaged wiring at the ignitor or control board will show as open circuits or intermittent voltage. Visual inspection and continuity checks reveal these problems.

Tools And Safety Before Testing Voltage

Testing for furnace ignitor voltage requires care. Always follow furnace manufacturer recommendations and local codes. Recommended tools include a multimeter with AC and DC settings, non-contact voltage tester, insulated screwdrivers, needle-nose pliers, and safety gloves and eye protection.

• Turn off power at the furnace switch and breaker before inspecting wiring.
• Use proper PPE and never bypass safety devices.
• When in doubt call a licensed HVAC technician for live-voltage diagnostics.

Step-By-Step Diagnostic Procedure

These steps guide a systematic approach when a furnace ignitor is not getting voltage. Perform checks in order to isolate the failure efficiently.

1. Confirm The Symptom

Set the thermostat to heat and increase temperature to initiate a call for heat. Observe furnace behavior: does the blower run, does the inducer motor start, are there any error LED codes on the control board? These clues narrow the problem area.

2. Verify Power To Furnace

Use a multimeter to verify 120VAC at the furnace’s power input. Check that the on/off switch and circuit breaker are on. No line voltage requires electrical troubleshooting rather than HVAC.

3. Check The Transformer Output

Measure the transformer’s secondary for ~24VAC between R and C. If the transformer is not producing voltage, it must be replaced. Some transformers can be tested under load; a weak transformer may show reduced voltage under load and fail to energize the ignitor.

4. Inspect The Control Board Fuse

Locate and inspect the 24VAC control fuse on the board. Replace a blown fuse with the correct rating and retest. If the fuse blows again immediately, do not repeatedly replace it—identify the short or fault causing the overcurrent.

5. Test For 24VAC At Ignitor Connector

With a heat call active, measure voltage at the ignitor harness. If 24VAC is present but the ignitor doesn’t glow, the ignitor itself is likely failed. If 24VAC is absent, trace backward to the control board output to find where voltage is lost.

6. Check Safety Switches And Pressure Switch

Measure continuity across limit switches and pressure switch terminals during a heat call. A pressure switch requires correct inducer operation and venting; if the inducer motor is running but the pressure switch remains open, inspect vent tubing and switch condition.

7. Evaluate Control Board Outputs

Using the furnace wiring diagram, verify that the control board is issuing a command to the ignitor output during the heat call. No command suggests a board logic fault or upstream issue like incorrect thermostat signal.

8. Inspect Wiring And Connectors

Look for pinched, burned, or corroded wires between the board, transformer, pressure switch, and ignitor. Repair or replace damaged wires and connectors, ensuring secure crimped or soldered connections.

How To Test And Replace An Ignitor

If 24VAC is present at the ignitor connector but the ignitor does not glow, the ignitor is likely failed. Hot surface ignitors are fragile; handle by the ceramic, not the heating element.

• Remove the ignitor and inspect for cracks or discoloration.
• Test ignitor resistance with an ohmmeter; typical HSI values vary by model but often range from 40 to 120 ohms. Consult the furnace manual.
• Replace the ignitor with OEM or equivalent part; install carefully and avoid touching the element with bare hands.

Common Control Board And Relay Failures

Control boards suffer from burned traces, failed relays, or connector issues. Boards exposed to heat, moisture, or voltage spikes are vulnerable. Symptoms include intermittent ignition, no voltage to ignitor, or diagnostic LED error codes. Replacing the board is often the permanent fix after ruling out wiring and safety switch faults.

When The Pressure Switch Or Inducer Is The Problem

The inducer motor creates venting draft that allows the pressure switch to close and enable the ignitor. If the inducer fails or venting is blocked, the pressure switch will stay open and prevent voltage to the ignitor. Clean or replace the inducer motor, clear blockages, and replace faulty pressure switches.

Costs And Timeframes For Repairs

Repair costs vary widely by region and furnace type. Typical ranges: ignitor replacement $100–$300, transformer $120–$350, control board $300–$800, pressure switch $80–$200, inducer motor $300–$700. Labor and emergency service fees increase total cost.

Simple fixes like replacing a fuse or ignitor often take under an hour. Troubleshooting complex electrical or venting issues may take several hours and require specialized parts and tools.

When To Call A Professional

Call a licensed HVAC technician when live-voltage testing is required, when the control board needs replacement, or when diagnosing complex safety switch and venting interactions. Also engage a professional if the furnace repeatedly blows fuses or if major components like the inducer motor fail.

Preventive Maintenance To Avoid Ignitor Voltage Issues

Regular maintenance reduces chances of a furnace ignitor not getting voltage. Recommended measures include annual professional tune-ups, replacing air filters every 1–3 months, keeping vents and flues clear, inspecting wiring during service, and monitoring error codes or unusual furnace behavior.

Troubleshooting Checklist

1. Confirm thermostat calls for heat and wiring R-W integrity.
2. Verify 120VAC power to furnace and circuit breaker status.
3. Measure transformer secondary for ~24VAC.
4. Inspect control board fuse and replace if blown.
5. Check for 24VAC at the ignitor harness during heat call.
6. Test continuity of limit, rollout, and pressure switches.
7. Inspect inducer operation and venting for blockages.
8. Evaluate control board outputs and relays for faults.
9. Replace ignitor if voltage present but element does not glow.
10. Call licensed HVAC technician for control board or complex electrical faults.

Safety Reminders And Code Considerations

Work on furnaces involves electrical and gas systems. Never bypass safety switches, and avoid DIY gas valve work. Follow National Fuel Gas Code and local jurisdictions for gas appliance servicing. Use a qualified technician for gas-related repairs and permits.

Key Takeaways

When a furnace ignitor is not getting voltage the issue usually lies upstream: transformer failure, blown control fuse, safety switches, control board faults, or wiring problems. Systematic testing—verifying power, transformer output, fuse, safety switches, and control board commands—identifies the cause. Replace the ignitor only after confirming the control circuit supplies voltage. For live-voltage tests or control board replacement, engage a licensed HVAC professional to ensure safety and code compliance.

Further Resources: Refer to the furnace owner’s manual for wiring diagrams and part numbers, consult the National Fuel Gas Code for safety practices, and use manufacturer troubleshooting guides for model-specific diagnostics.