Why a Furnace Is Intermittently Not Working and How to Fix It

The intermittent failure of a furnace can be disruptive and costly. This article explains common causes, diagnostic steps, safety considerations, and practical fixes for a furnace intermittently not working, providing homeowners with clear, actionable guidance to restore reliable heating and decide when to call a professional.

Common Symptom	Likely Cause	Suggested Action
Runs Then Stops	Overheating/limit switch or airflow problem	Check filters, vents, blower; clean or replace filter
Brief Ignition Failures	Igniter or flame sensor issues	Inspect/clean flame sensor; test igniter
No Heat After Thermostat Call	Thermostat, control board, or power issue	Verify thermostat settings; check breaker and power

How Intermittent Furnace Problems Manifest

A furnace intermittently not working may show several patterns: it starts and stops repeatedly, runs for short cycles, fails to ignite sometimes, or works only on certain thermostat settings. Recognizing the pattern helps narrow down causes between electrical, fuel, combustion, ventilation, and control-system issues.

Safety First: Precautions Before Troubleshooting

Before any inspection, the furnace’s power should be shut off at the breaker and the gas supply turned off if working on a gas furnace. If there is any suspicion of a gas leak, leave the property and call the gas company or emergency services. When in doubt, contact a licensed HVAC technician to avoid risk of fire, carbon monoxide, or electrical injury.

Common Causes And Diagnostic Steps

1. Airflow Problems And Overheating

Restricted airflow causes the furnace to overheat and trip the high-limit safety switch, resulting in short cycles or shutdowns. Common airflow issues include dirty air filters, blocked supply or return vents, closed dampers, or blower motor problems. Check and replace the air filter, ensure vents are open, and inspect the blower compartment for debris.

2. Thermostat And Control Signals

Faulty thermostats, incorrect settings, or wiring problems can cause a furnace intermittently not working. Verify thermostat mode is set to “Heat”, confirm temperature setpoint is higher than room temperature, and replace batteries if applicable. Use a multimeter to check for 24V signal at the furnace control board if thermostat troubleshooting is beyond basic checks.

3. Ignition And Flame Detection Issues

Modern furnaces use hot-surface igniters or intermittent pilots and a flame sensor to confirm combustion. An intermittently failing igniter or a dirty flame sensor will produce inconsistent ignition. Clean the flame sensor with fine emery cloth and visually inspect the igniter for cracks or glaze; replace if damaged.

4. Gas Supply And Pressure Problems

Insufficient gas pressure, a partly closed gas valve, or a blocked gas line can cause intermittent operation. Symptoms include ignition that starts and then goes out or weak burners. If gas supply is suspected, contact the gas utility or a professional technician to measure pressure and inspect valves—handling gas systems without training is unsafe.

5. Electrical And Control Board Failures

Loose wiring, failing relays, blown fuses, or a malfunctioning control board may interrupt furnace operation. Inspect electrical connections for tightness and signs of burning. Check auxiliary fuses on the control board; a repeated fuse failure indicates a deeper electrical fault that requires professional diagnosis.

6. Blower Motor And Capacitor Problems

If the blower motor or its capacitor is failing, the furnace may not circulate heat properly, causing overheating or short cycling. Listen for humming, grinding, or intermittent blower starts. Capacitors can be tested and replaced, but electrical work should be performed with power disconnected and by someone comfortable with high-voltage components.

7. Limit Switch And Safety Sensors

Limit switches monitor temperature and can shut down the burner if readings are out of range. A malfunctioning or miscalibrated limit switch, or buildup around the heat exchanger, can cause intermittent shutdowns. Cleaning the heat exchanger area and verifying proper operation of safety sensors can resolve these issues, but sensor replacement often requires professional service.

Step-By-Step Troubleshooting Guide

1. Confirm Power And Gas: Ensure circuit breaker is on, furnace switch is on, and gas valve is open.
2. Check Thermostat: Replace batteries, verify mode and setpoint, and test with a basic spare thermostat if available.
3. Replace Or Clean Filter: Install a recommended MERV-rated filter and note whether problem improves.
4. Inspect Ignition System: Observe ignition sequence; clean flame sensor and check igniter visually.
5. Examine Vents And Ducts: Remove obstructions, check for closed dampers, and ensure attic/ crawlspace intake vents are clear.
6. Listen For Abnormal Sounds: Hums, clicks, or bangs help identify motor, relay, or combustion problems.
7. Check Error Codes: Many furnaces blink LED codes on the control board; record blinking patterns and consult manufacturer literature.

When To Call A Professional HVAC Technician

Some intermittent problems require diagnostics and tools beyond homeowner capability. Call a licensed HVAC technician if there are persistent ignition failures, suspected gas leaks, recurring electrical fuse blows, damaged heat exchangers, or repeated safety shutdowns. Professionals can perform combustion analysis, measure gas pressure, test control boards, and safely replace components.

Maintenance Tips To Prevent Intermittent Failures

• Replace Filters Regularly: Change or clean air filters every 1–3 months depending on usage and pets.
• Schedule Annual Tune-Ups: Annual inspection and cleaning by a licensed technician reduce unexpected malfunctions.
• Keep Vents And Returns Clear: Maintain unobstructed airflow throughout the home and ensure return grills are open.
• Inspect Flue And Venting: Ensure exhaust flues are unobstructed and caulking around vent joints is intact.
• Watch For Rust Or Corrosion: Corrosion can indicate condensate or venting issues that lead to intermittent operation.

Cost Considerations And Typical Repairs

Repair costs vary widely based on the faulty component. Simple tasks like filter replacement or flame-sensor cleaning are low cost. Parts such as igniters, capacitors, or thermostats are moderate in price with reasonable labor. Replacing a control board, heat exchanger, or performing major gas-system repairs can be expensive and may influence decisions about replacement versus repair.

Repair	Typical Cost Range
Filter Replacement	$10–$60
Flame Sensor Cleaning/Replacement	$50–$200
Igniter Replacement	$100–$400
Blower Motor/Capacitor	$150–$800
Control Board Replacement	$300–$1,000+

How To Interpret Furnace Error Codes

Many furnaces use LED blink codes to indicate faults. Blink patterns correspond to issues like pressure switch failure, ignition lockout, or limit trips. Locate the control board panel, count blinks, and consult the furnace manual or manufacturer website. Recording the code and runtime context helps technicians diagnose intermittent problems faster.

Special Considerations For Older Furnaces

Older furnaces may experience intermittent failures due to wear, obsolete parts, clogged burners, and degraded heat exchangers. Efficiency and safety standards have advanced; repeated, costly repairs on an older unit might justify replacement. Evaluate age, repair frequency, and the presence of safety issues such as cracked heat exchangers before deciding.

Signs Of Serious Problems Requiring Immediate Action

• Smell Of Gas Or Rotten Eggs: Evacuate and call the gas company or emergency line.
• Persistent Carbon Monoxide Alarm: Leave the home and call emergency services and HVAC professionals.
• Visible Cracks In The Heat Exchanger: This presents a carbon monoxide hazard and demands immediate furnace shutdown.
• Frequent Short Cycling With No Improvement: May indicate severe airflow, control, or combustion issues requiring professional repair.

Useful Tools And When To Use Them

Homeowners can safely use a few tools for basic diagnostics: a reliable multimeter for checking voltages and thermostat signals, a manometer for gas pressure (professionals typically use this), a combustion analyzer (technician), and a small mirror/flashlight for visual inspections. Personal protective equipment and proper shutoff procedures are essential when using tools.

Summary And Next Steps

If a furnace is intermittently not working, begin with simple checks: power, thermostat, and filter, then observe ignition and blower behavior. Document patterns, LED error codes, and noises. If problems persist or if any safety red flags appear, contact a licensed HVAC technician for a thorough inspection and repair. Regular maintenance is the most effective long-term strategy to prevent intermittent failures and ensure safe, efficient operation.