Why Is My Furnace Turning on but Not Igniting

The furnace turning on but not igniting can leave a home cold and create safety concerns. This article explains common causes, step-by-step troubleshooting, and safe fixes for gas and electric furnaces to help homeowners identify the problem and decide when to call a technician. Key focus: ignition systems, pilot and igniter faults, safety switches, gas supply, and maintenance tips.

Symptom	Likely Cause	Action
Fan/blower runs but no heat	Thermostat signal, limit switch, or control board	Check thermostat settings; inspect error codes; test breakers
Clicking but no flame	Igniter or pilot issue, gas valve problem	Inspect igniter; verify gas supply; replace igniter or call pro
Brief flame then shuts off	Dirty flame sensor or bad gas pressure	Clean flame sensor; test gas valve and pressure

How A Furnace Ignition System Works

A basic understanding helps isolate problems quickly. Modern gas furnaces use either a standing pilot, intermittent pilot, or hot-surface/induced spark igniter to light the burners. The thermostat calls for heat, the inducer and blower may run to establish draft, the gas valve opens when the ignition device is active, and the flame sensor confirms combustion. Control boards manage the sequence and safety switches monitor pressure, temperature, and airflow.

Common Causes When The Furnace Turns On But Won’t Ignite

Multiple components can cause the symptom where the furnace powers up but fails to light. Identifying whether the system attempts ignition or simply powers the fan is the first diagnostic step. Below are the most frequent causes seen in residential systems.

Thermostat Or Control Signal Issues

If the thermostat isn’t sending a proper heat call, the furnace may run fans or inducer motors due to a miswired or failing control board while not initiating the ignition sequence. Check thermostat mode, setpoint, and wiring. Replace batteries if applicable and verify the heat call connection at the furnace control board.

Igniter Problems (Hot Surface Or Spark)

Hot-surface igniters can crack or burn out, and spark igniters can fail from electrode wear or loose wiring. A failed igniter often results in repeated clicking or a complete absence of ignition attempts. Signs include no glow on hot-surface igniters or continuous click with no flame.

Pilot Light Or Pilot Assembly (Older Furnaces)

Furnaces with a standing pilot may have a clogged pilot or thermocouple issue. If the pilot won’t stay lit or the thermocouple fails to sense heat, the gas valve will remain closed. Pilot maintenance and thermocouple testing are common solutions for older units.

Flame Sensor Fouling

A dirty flame sensor may not detect a flame and will shut off the gas after ignition. This often causes the burners to light briefly then extinguish. Cleaning the flame sensor with fine emery cloth can restore operation.

Gas Supply And Gas Valve Faults

No gas flow, low gas pressure, or a malfunctioning gas valve prevents ignition. A closed gas shutoff, interrupted supply from the utility, or a faulty valve coil can all be factors. Always verify the gas is on and consult the utility or a licensed technician for pressure or valve issues.

Safety Switches And Pressure Switch Problems

Negative pressure switches, high-limit switches, and rollout switches protect against unsafe conditions. If any safety device is stuck open or not closing during startup, the control board will inhibit ignition. Error codes on the furnace board or diagnostic LED flashes often indicate which switch triggered.

Blocked Venting Or Inducer Motor Failure

Induced draft furnaces require the inducer motor to create proper vent pressure before ignition. A blocked flue, damaged inducer, or clogged condensate trap can prevent the pressure switch from closing and stop ignition. Inspect vents for obstructions and ensure the inducer motor runs.

Step-By-Step Troubleshooting Checklist

Follow these safe, logical steps to narrow the root cause. If the homeowner is uncomfortable performing any step, they should stop and call a licensed HVAC technician. Safety first: turn power off before opening the furnace panel.

1. Verify Thermostat Settings: Set to heat, raise setpoint above room temperature, and confirm the furnace receives the call by checking control board LEDs or listening for a start sequence.

2. Check Power And Gas Supply: Confirm circuit breaker is on, furnace switch is on, and gas shutoff valve near the furnace is open. Smell for gas only if confident and safe; if gas odor is present, leave immediately and call the gas company.

3. Listen For Ignition Attempts: Note if there’s clicking, a glow from the igniter, or brief flame. This helps determine if the igniter works or if gas flow is the issue.

4. Inspect Error Codes And LEDs: Many systems flash diagnostic codes on the control board. Record the pattern and consult the furnace manual to interpret the code.

5. Examine The Flame Sensor: If burners light briefly then go out, remove and gently clean the flame sensor with fine sandpaper or emery cloth. Reinstall and test.

6. Check The Igniter: For hot-surface igniters, look for cracks or discoloration. For spark igniters, inspect electrodes and wiring. Replace igniter if physical damage or no continuity.

7. Inspect Venting And Inducer: Ensure intake and exhaust vent pipes are clear. Observe the inducer motor for proper operation at startup.

8. Test Pressure And Limit Switches: Visually inspect hoses and switches for disconnections or soot. A multimeter can test continuity if comfortable using one.

9. Call A Professional If Needed: Contact an NATE-certified technician for gas valve testing, manifold pressure checks, or control board replacement.

How To Safely Clean Or Replace Common Parts

Some maintenance is homeowner-friendly; other tasks require professional tools. Always shut power and gas off before servicing a furnace and follow manufacturer instructions.

Cleaning The Flame Sensor

Remove the sensor, wipe gently with fine emery cloth to remove carbon build-up, avoid scraping or bending, reinstall, and test. This simple step often resolves intermittent ignition issues.

Replacing A Hot Surface Igniter

Order the exact OEM replacement for the furnace model. Replace the igniter carefully as it is brittle. Replacing an igniter is a manageable DIY if comfortable with basic disassembly and electrical connectors.

Inspecting And Resetting Safety Switches

Pressure switch hoses can get disconnected or clogged with moisture. Clear condensate traps, reconnect hoses, and reset switches by restoring power. Do not bypass safety switches; calling a technician is recommended if switches repeatedly trip.

When To Call A Professional

Contact a licensed HVAC or gas technician if any of these apply: persistent ignition failure after basic checks, suspected gas leak, failed gas valve, repeated safety switch trips, control board errors, or lack of confidence performing diagnostics. Professionals have tools to measure gas pressure, test control voltages, and safely replace components.

Preventive Maintenance To Reduce Future Ignition Problems

Routine maintenance improves reliability and safety. Annual or biannual tune-ups by a qualified technician prevent many ignition problems and optimize efficiency. Key maintenance items include filter changes, burner and sensor cleaning, vent inspection, and electrical checks.

• Change Air Filters every 1–3 months depending on system and home conditions to maintain airflow and prevent overheating.

• Schedule Annual HVAC Service to inspect ignition components, test gas pressure, and clean burners and flame sensors.

• Keep Vents Clear of debris, snow, or nesting animals that can block intake or exhaust pipes.

• Monitor Thermostat Batteries and calibration to avoid incorrect heat calls that confuse the system sequence.

Safety Tips And Red Flags

Never attempt gas valve or manifold pressure work without certification. If a strong gas odor occurs, evacuate and call the gas company. Avoid bypassing safety switches or jump-starting control circuits. Repeated ignition failures may indicate dangerous conditions requiring immediate professional attention.

Common Diagnostic Codes And What They Mean

Many modern furnaces display flash codes. While codes vary by manufacturer, common meanings include pressure switch failure, limit switch open, flame loss, igniter failure, and control board faults. Always consult the furnace manual or a technician to interpret and act on codes safely.

Cost Considerations For Repairs

Repair costs vary widely by component and region. Typical ranges: flame sensor cleaning often low cost or DIY, igniter replacement $100–$400 including parts and labor, gas valve replacement $300–$800, control board replacement $300–$1,000. Emergency service and diagnostic fees add to the total. Obtain multiple estimates for major repairs.

Frequently Asked Questions

Why Does The Furnace Fan Run But No Heat?

The fan may run because the thermostat supplies power to the blower or the control board is in a diagnostic mode. This can occur when ignition is inhibited by a safety device or an igniter failure prevents burner operation.

Can A Dirty Filter Prevent Ignition?

A clogged filter can cause heat buildup, tripping the high-limit switch and preventing ignition until the system cools. Replace filters regularly to avoid airflow-related shutdowns.

Is It Safe To Try Lighting The Pilot Manually?

Manual pilot lighting is only appropriate for older furnaces with standing pilot lights and when the homeowner is familiar with the procedure. If unsure, contact a professional to avoid gas buildup and ignition hazards.

Resources And Next Steps

Homeowners should consult their furnace manual for model-specific diagnostics and note any diagnostic LED patterns before contacting a technician. Keep records of service, part replacements, and error codes to help professionals diagnose recurring issues faster.