Furnace Flame Goes Out When Blower Comes On: Causes and Fixes

The furnace flame going out when the blower comes on is a common HVAC problem that can stem from electrical, airflow, or safety-control issues. This article explains why the flame goes out when the blower starts, outlines diagnostic steps, and lists repairs and preventive measures homeowners can use to resolve the issue safely.

Symptom	Possible Causes	Common Fixes
Flame Extinguishes When Blower Engages	Faulty pressure switch, weak flame sensor, blocked vent, failing transformer, limit switch trip	Clean sensor, repair venting, replace pressure switch/transformer, professional inspection

How Furnaces Control Flame And Blower Operation

Modern gas furnaces coordinate flame and blower operation using control boards, safety switches, and sensors. The sequence generally includes thermostat call, ignition, flame establishment, blower on delay, and blower activation after heat exchanger warms.

The blower is interlocked with safety devices—such as the pressure switch, flame sensor, and limit switch—that will shut off the gas if conditions indicate a fault or unsafe combustion.

Common Reasons The Flame Goes Out When The Blower Starts

1. Faulty Pressure Switch Or Venting Problems

The pressure switch confirms the inducer motor created proper draft before ignition. If the venting is blocked or the pressure switch is marginally failing, the blower’s airflow change can drop draft or create a brief pressure change that trips the switch and shuts off the gas.

2. Weak Or Dirty Flame Sensor

The flame sensor detects continuous combustion. When the blower changes airflow or vibration occurs, a weak or contaminated flame sensor may momentarily fail to sense the flame and the control board will shut off the gas as a safety measure.

3. Limit Switch Or Overheat Condition

If the heat exchanger or plenum is near overheating, the limit switch can trip when blower operation changes temperatures and airflow. A tripped limit switch will extinguish the flame to prevent damage or unsafe temperatures.

4. Electrical Issues: Transformer, Relay, Or Control Board

Voltage drops or failing transformers and relays can manifest when the blower motor engages. If the control board does not receive stable 24VAC signals or the blower relay shorts, the gas valve may close and the flame will go out.

5. Inducer Motor Or Blower Motor Load Problems

A failing inducer or blower motor can draw excessive current or create abnormal pressures when it runs. This can trip safety circuits or cause the flame to extinguish during the transition from ignition to blower operation.

Symptoms To Observe During A Failure

• Flame Present During Ignition But Goes Out When Blower Starts
• Diagnostic LED error codes on the furnace control board
• Inducer or blower unusually noisy or slow to spin
• Periodic trips of the limit or pressure switch
• Intermittent operation that sometimes runs normally

Step-By-Step Troubleshooting Checklist

Use this checklist as a guide. Turn off power and gas before performing any work that requires opening panels or handling wiring.

Visual And Basic Checks

• Inspect vents and exhaust for blockages, bird nests, or ice buildup that would affect draft.
• Confirm air filter is clean and return airflow is not restricted.
• Look for error codes on the furnace LED and record blinking patterns for reference.

Test The Flame Sensor

• Turn off power and remove the flame sensor. Clean it gently with fine emery cloth or steel wool and rinse with alcohol. Reinstall and test operation.
• If cleaning does not help, replace the flame sensor. This is a common, low-cost repair that often resolves flame dropout issues.

Check The Pressure Switch And Vent Pressure

• Inspect the pressure tubing for cracks or disconnection. Replace brittle tubing.
• Use a multimeter or manometer to verify the pressure switch closes properly when the inducer runs.
• Replace a failing pressure switch after confirming venting is clear.

Evaluate Limit Switch And Heat Exchanger Temperatures

• Check limit switch wiring continuity with power off. A tripped or failing limit switch may need replacement.
• If overheating is suspected, investigate airflow issues, clogged coils (in heat pumps), or a failing blower motor.

Inspect Electrical Components

• Measure transformer output to ensure stable 24VAC when the blower engages.
• Check blower relay contacts and control board connections. Swollen capacitors, burnt traces, or loose wiring can cause intermittent shutdowns.

Listen For Motor Strain Or Abnormal Operation

• A noisy or struggling blower may indicate bearings or motor failure. Motor issues can alter electrical load and trip safety circuits.

When To Call A Professional HVAC Technician

Some troubleshooting steps are safe for homeowners, but safety systems and gas components demand professional service in many cases. Contact a licensed technician if any of the following apply:

• Any gas line or valve work is required.
• Control board, transformer, or gas valve replacement is needed.
• Heat exchanger cracks, corrosion, or suspected carbon monoxide risk exist.
• Complex electrical diagnostics or motor replacements are necessary.

Repair Options And Typical Costs

Costs vary by region and furnace model. Approximate ranges for common repairs are provided as a guide.

Repair	Typical Cost Range (USD)
Flame Sensor Cleaning/Replacement	$75–$200
Pressure Switch Replacement	$150–$350
Limit Switch Replacement	$120–$300
Blower Motor Replacement	$400–$900
Control Board Replacement	$300–$800

These estimates include labor; exact pricing depends on accessibility, part quality, and local labor rates. A qualified technician can provide an accurate quote after diagnosis.

Safety Considerations And Carbon Monoxide Risk

A furnace that repeatedly shuts off the flame may signal combustion or venting issues. Always ensure carbon monoxide detectors are operational and never ignore persistent shutdowns or smells of gas.

Shutting off the furnace and calling a professional is appropriate if any gas odor, persistent error codes, or suspected heat exchanger damage is present.

Preventive Maintenance To Reduce Recurrence

• Change or clean air filters every 1–3 months during heavy use to maintain airflow.
• Schedule annual professional tune-ups to inspect sensors, safety switches, heat exchanger condition, and electrical components.
• Keep exhaust vents, chimneys, and intake areas clear from debris, bird nests, or snow/ice.
• Install or test carbon monoxide detectors on each floor and replace batteries regularly.

What Homeowners Should Tell A Technician

When contacting an HVAC professional, provide clear information to speed diagnosis: thermostat settings, when the flame goes out (immediately when blower starts or after delay), any error codes, recent maintenance history, and whether the issue is intermittent or consistent.

Key Takeaways And Practical Next Steps

Key Points: Flame dropout when the blower engages usually ties to pressure switch and venting issues, flame sensor contamination, limit switch trips, or electrical faults. Small fixes like cleaning the flame sensor or replacing tubing can resolve many cases; other repairs require a technician.

Practical Next Steps: 1) Replace dirty air filter; 2) Clean flame sensor; 3) Inspect vents; 4) Note control-board error codes; 5) Call a licensed HVAC technician if the problem persists or if any gas/CO concerns exist.

Addressing the issue promptly protects safety, improves furnace efficiency, and helps avoid more costly repairs down the line.