Furnace Cycling but Not Igniting: Causes, Troubleshooting, and Fixes

The article explains why a furnace may cycle but not ignite and provides clear troubleshooting steps, safety tips, and repair options for homeowners. It focuses on diagnosing common causes such as ignition system faults, flame sensor issues, gas supply problems, and control board or blower interactions while emphasizing safe practices and when to call a professional.

Symptom	Likely Cause	Quick Action
Furnace Blows Cold Air Then Tries To Ignite	Dirty Flame Sensor Or Weak Igniter	Clean Flame Sensor, Observe Igniter
Furnace Starts Fan But No Ignition	Limit Switch Or Control Board Fault	Check Error Codes, Reset Power
Multiple Attempts To Light Then Lockout	Gas Supply Or Pressure Issue	Verify Gas Valve, Call Gas Company

How Furnaces Normally Ignite

Modern forced-air furnaces use an ignition sequence controlled by the thermostat and the furnace control board. When heat is needed, the sequence typically includes starting the inducer motor, confirming pressure switch closure, energizing the hot surface igniter or spark, opening the gas valve, and sensing a flame via the flame sensor.

Understanding this sequence helps pinpoint where ignition fails. If the furnace cycles through parts of the sequence but never establishes a flame, the fault is usually in one of the ignition components, the gas delivery, or a safety interlock.

Common Causes When A Furnace Cycles But Does Not Ignite

Faulty Igniter (Hot Surface Or Spark)

The igniter is responsible for lighting the gas. Hot surface igniters can crack or become weak, and spark igniters can fail to produce a reliable spark. Symptoms include visible glow failure or no spark during the ignition attempt.

Testing: Visual inspection for cracks on a hot surface igniter, and observation for a visible spark on spark systems. Multimeter testing for ohms can indicate continuity issues for hot surface igniters.

Dirty Or Failing Flame Sensor

The flame sensor detects whether the burners have lit and prevents gas flow if no flame is present. A dirty flame sensor often allows ignition attempts but fails to sense the flame, causing repeated cycling and eventual lockout.

Cleaning the flame sensor with fine steel wool or emery cloth often resolves this. Do not use abrasive methods that change the sensor shape. Tighten gently after cleaning.

Gas Supply And Gas Valve Problems

If the furnace cycles through the ignition sequence but there is no gas flow, the gas valve, gas line, or external supply may be the issue. The furnace may attempt ignition multiple times and then enter a safety lockout.

Check other gas appliances to see if gas is available. Any suspected gas leak or supply interruption requires contacting the gas utility or a licensed technician due to safety risks.

Pressure Switch And Venting Issues

The pressure switch confirms proper inducer motor operation and clear venting. If the switch fails to close, the furnace will not proceed to ignition. Blocked vents, a failed inducer motor, or a stuck pressure switch can cause cycling without ignition.

Inspect for blocked vents, bird nests, or damaged PVC intake/exhaust. A multimeter can verify pressure switch continuity when the inducer runs.

Control Board, Limit Switches, And Safety Interlocks

The control board orchestrates the ignition sequence. Faulty relays or sensors like the high-limit switch can prevent gas valve opening. Shorted or open safety switches may cause the furnace to cycle the blower or inducer but not ignite.

Look for diagnostic LED codes on the control board. Many furnaces flash an error sequence that helps identify the failing component.

Thermostat And Wiring Issues

A thermostat that sends intermittent or incorrect signals can cause repeated cycling without completing ignition. Wiring shorts, loose connections, or an incorrect fan setting may also interfere with the ignition sequence.

Test by setting the thermostat to call for heat and listening for the normal sequence. Inspect visible wiring for damage and ensure terminals are secure.

Troubleshooting Steps Homeowners Can Safely Perform

Step 1: Observe And Record Behavior

Note what the furnace does when the thermostat calls for heat: does the inducer motor run, does the igniter glow or spark, does the gas valve click? Recording the behavior and any error code flashes is valuable information.

Step 2: Check For Error Codes And Reset Power

Locate the control board and read the LED blink pattern or digital code. Consult the furnace manual for code definitions. A simple power reset (turn off power for 30 seconds) can clear temporary faults, but repeated resets are not a fix.

Step 3: Clean The Flame Sensor

Turn off power and gas, remove the flame sensor, and clean it gently with emery cloth or fine sandpaper to remove oxidation. Reinstall and test. This fix often resolves cycling with repeated ignition attempts.

Step 4: Inspect The Igniter

Visually inspect a hot surface igniter for cracks or a spark igniter for visible spark. Replace igniters that are damaged. Do not touch hot surface igniters with bare hands—oils reduce life span.

Step 5: Check Vents And Pressure Switch Tubing

Examine intake and exhaust vents for blockage and inspect the small vacuum tubing to the pressure switch for disconnections or cracks. Clear obstructions and secure tubing before testing the furnace again.

Step 6: Verify Gas Supply

Confirm the gas supply is on and that other gas appliances operate. If the furnace’s gas valve does not open but other appliances have gas, the valve or control board may be faulty.

When To Stop And Call A Professional

If troubleshooting does not restore ignition, or any testing involves gas valve replacement, control board repair, or suspected gas leaks, stop and contact a licensed HVAC technician. Working on gas and electrical components poses safety risks and may void warranties if done incorrectly.

Call a professional immediately if there is any smell of gas. Evacuate the home and contact the gas company from a safe location.

Diagnostic Codes And What They Typically Mean

Many furnaces provide LED blink codes that identify component failures. Common codes include pressure switch failures, ignition failure, flame loss, and limit switch trips. The exact meaning varies by manufacturer.

Document the blink pattern and consult the furnace manual or the manufacturer’s website. Technicians use these codes to fast-track diagnosis and parts replacement.

Costs And Typical Repairs

Repair costs vary by component. Cleaning a flame sensor is often low cost if done by a technician. Igniter replacement ranges from moderate to higher cost depending on model. Control board replacement is typically more expensive.

Labor rates and parts vary regionally. Homeowners should get written estimates and verify technician credentials and references before authorizing repairs.

Maintenance Tips To Prevent Cycling Without Ignition

• Schedule annual professional furnace tune-ups to clean burners, inspect ignition components, and test safety controls.
• Replace air filters every 1–3 months to maintain airflow and reduce stress on safety limits.
• Keep intake and exhaust vents clear of debris, snow, or nesting material.
• Have a licensed technician inspect gas lines and pressure annually or if appliance performance changes.

Safety Precautions While Troubleshooting

Always shut off power to the furnace at the service switch or breaker and shut off the gas when working on ignition components unless performing visual observation only. Use a multimeter only if comfortable and trained in basic electrical safety.

If the homeowner detects a gas odor, leave the premises immediately and contact the gas utility or emergency services. Avoid operating light switches or electronics that could ignite leaked gas.

Replacement Versus Repair: Making The Decision

Repair is cost-effective for single-component failures like an igniter or flame sensor. Consider replacement if the furnace is older than 15–20 years, requires frequent repairs, or has declining efficiency. Newer systems offer improved safety, efficiency, and smart thermostat compatibility.

Compare estimated repair cost versus replacement cost and factor in energy savings, warranty, and expected remaining service life when deciding.

Frequently Asked Questions

Why Does The Furnace Try To Ignite Multiple Times Then Lock Out?

Repeated attempts followed by lockout typically indicate the furnace detects no flame after multiple tries. This can be due to a dirty flame sensor, weak igniter, gas valve not opening, or intermittent flame that the sensor doesn’t detect.

Can A Dirty Air Filter Cause Cycling Without Ignition?

A clogged filter can cause limit trips and abnormal cycling but usually does not prevent ignition entirely. However, restricted airflow can heat internal safeties and confuse the sequence, so maintaining clean filters is important.

Is It Safe To Manually Light A Furnace?

Manual lighting is not recommended for modern furnaces with electronic ignition. It can be dangerous and interfere with safety controls. Follow manufacturer guidance and call a technician if the automatic ignition fails.

Useful Tools And Parts For Technicians And Handy Homeowners

• Multimeter for continuity and voltage checks
• Replacement hot surface igniter or spark igniter compatible with the furnace model
• Fine emery cloth or steel wool for cleaning the flame sensor
• Infrared thermometer to verify burner temperatures and limit switch behavior
• Manufacturer service manual and control board error code reference

Key Takeaways

Furnace cycling but not igniting is most often caused by a dirty flame sensor, a failing igniter, gas supply issues, pressure switch or venting problems, or safety interlocks and control board faults. Safe, methodical troubleshooting can resolve many issues, but any suspected gas leak, complex electrical diagnosis, or control board repair should be handled by a licensed HVAC technician.