HVAC SENIOR

There are few things more frustrating than a furnace that refuses to stay lit when you need heat the most. You’ll hear the system start, the burners will fire on for a moment, and then abruptly it all goes out. The result is cold air coming through the vents and higher energy bills.

This common HVAC problem usually indicates an issue with either ignition, flame detection, or airflow. However, there’s no need to panic because most of the reasons that could cause this malfunction can be diagnosed and repaired if properly approached. In this article, we will discuss why your furnace is not staying lit, how to troubleshoot the problem step by step, and when to call in a professional.

How a Furnace Flame Works

Before delving into the causes, let’s first understand the basic furnace ignition process:

• The thermostat signals the furnace to start.

• It clears out the exhaust gases from the combustion chamber.

• It is the igniter-spark or hot surface-that lights the gas issuing from the burners.

• The flame sensor ensures a stable flame is present.

• The main gas valve remains open as long as the flame is detected.

If anything in this chain fails for even a second, as a safety precaution, the control board shuts off the gas, and your furnace flame goes out.

Also read: Furnace Short Cycling

Common Reasons Why a Furnace Won’t Stay Lit

Here are the top 10 causes of a furnace that ignites but won’t keep burning.

1. Flame Sensor Either Dirty or Faulty

The flame sensor is a small metal rod that detects the presence of a flame. If it is dirty or corroded, it cannot sense the flame properly, which then causes the system to shut off the gas within seconds.

Signs:

• Furnace lights for a second then goes out

• Repeated short heating cycles

• Yellow, or weak flame

Solution:

Turn off the power and gas, take out the flame sensor, and clean it with fine steel wool or emery cloth. Put it back on carefully. If cleaning doesn’t help, the sensor should be replaced.

2. Clogged Air Filter

When the air filter becomes dirty, airflow is reduced and causes overheating in the furnace. The system trips the high-limit switch to prevent damage, shutting off the burners.

Solution:

Replace air filters every 1–3 months during heating season. Use filters with a MERV rating recommended by the manufacturer.

3. Faulty Igniter

A cracked or burned-out hot surface igniter will result in improper lighting of the gas and will, therefore, cause short or no cycles.

Solutions:

Test the igniter for continuity using a multimeter. If defective, replace it. Handle with care — igniters are fragile and sensitive to oils from your fingers.

4. Gas Valve Issues

The gas valve regulates the flow of fuel to the burners. If it’s stuck or malfunctioning, your furnace might ignite and then lose flame due to irregular gas pressure.

Solution:

Have a qualified technician test and replace the gas valve if needed. Never try to adjust or open the valve yourself — it’s a major safety risk.

5. Clogged or Filthy Burners

Dust, soot, or rust on the burner assembly can prevent proper flame spread. If one burner doesn’t ignite fully, the flame sensor may not detect a stable flame and will shut down the furnace.

Fix:

Turn off power and gas, then take out the burners and clean them with compressed air or a soft brush. The flame pattern should be blue and even.

6. Faulty Pressure Switch

The pressure switch ensures the safe venting of combustion gases. In case it fails or when the tubing is blocked, the control board may shut off the gas as a precautionary measure.

Fix:

Check the tubing or vent pipe for blockages. Replace the pressure switch if it does not properly close when the inducer motor is running.

Also read: Heating System Blowing Cold Air

7. Draft Inducer Motor Issues

The inducer draft draws combustion gases out of the furnace and develops the right flow needed at ignition. If it is weak or noisy, the system won’t be able to sustain a flame.

Correct:

Check for blockages, loose wiring, or worn bearings. Apply lubrication where possible or replace the motor if faulty.

8. Faulty Thermostat

Sometimes, the problem is at the thermostat and not the furnace. If it sends irregular signals, or if it loses calibration, the furnace may just ‘prematurely’ turn off.

Fix:

Replace old batteries, check the temperature setting, and recalibrate or replace the thermostat when necessary.

9. Blocked Flue or Vent

A blocked exhaust vent prevents combustion gases from escaping, triggering safety sensors to shut down the burners. Blockages can be caused by birds, debris, or snow.

Fix:

Inspect the vent termination outside your home and clear any obstruction. Proper ventilation is one of the fundamentals of safe furnace operation.

10. Defective Control Board

The control board is the brain of your furnace. If it starts to misread signals from sensors and switches, it may shut off the gas supply even when everything else works fine.

Verify:

In case of malfunction, the board should be tested and replaced by a professional technician. This component requires advanced diagnostics.

Also read: Heat Pump Not Defrosting

How to Troubleshoot a Furnace That Won’t Stay Lit

You can do a number of safe checks before calling in a professional. Here’s a step-by-step guide.

Step 1: Turn Off the Power and Gas

Safety first: Shut down your furnace before you begin examining internal parts.

Step 2: Check the Air Filter

Remove the filter and replace it if dirty. A clogged filter is one of the simplest yet most common culprits.

Step 3: Inspect the Flame Sensor

Remove the flame sensor and clean it with fine steel wool. Make sure it’s positioned properly near the burner flame.

Step 4: The Ignition Cycle Observation

Now turn the furnace back on and observe the process. If it lights and then shuts off after 3–10 seconds, the flame sensor is probably the culprit.

Step 5: Check Venting and Air Intake

Go outside and make sure the vent pipes are clear of leaves, ice, or animal nests.

Step 6: Reset the Furnace

Some models have a reset button — usually red — on the blower or control panel. Press it once, and see whether the furnace stays lit.

Step 7: Call a Professional

If the problem reoccurs, it would suggest a full failure of a gas valve, pressure switch, or control board that necessitates licensed repair.

Also read: Oil Furnace Not Starting

Preventive Maintenance Tips

Regular maintenance keeps your furnace reliable and extends its life. Here’s what to do:

• Schedule annual furnace tune-ups prior to winter.
• Regularly replace filters.
• Keep vents and intakes clear.
• Check and clean the flame sensor annually.
• Listen for unusual sounds coming from either the blower or inducer motor.
• Make sure your thermostat is calibrated.

Preventive maintenance is far cheaper than emergency repairs — and helps keep your home safe and warm.

Safety Warning

Never try to repair or replace gas parts unless you have proper training and certification. Gas leaks and carbon monoxide exposure are serious hazards. If you smell gas or suspect a leak, shut off the furnace immediately and call your gas provider or emergency services.

Professional Repair Costs

Here’s what you can expect to pay for common repairs when your furnace won’t stay lit:

These costs vary by brand, furnace type, and local service rates — but catching problems early always saves money.

When to Replace Your Furnace

 If your furnace is more than 15 years old, needs to be repaired often, or has a cracked heat exchanger, replacement could be the more cost-effective and safer option. New, high-efficiency furnaces (AFUE ratings of 90% or higher) can save up to 30 percent on energy bills.

Conclusion

A furnace that won’t stay lit is one of the most common but preventable heating problems that homeowners face. The cause could be as simple as a dirty flame sensor or as complex as a failing gas valve. With regular maintenance, timely cleaning, and professional inspections, you can keep your system running efficiently through every cold season. Don’t ignore the early warning signs. If your furnace continues to shut off, don’t wait anymore-a small fix today can save you from a costly breakdown tomorrow.