Furnace Lights for 5 Seconds Then Shuts Off: Causes and Fixes

The furnace lights for 5 seconds then shuts off is a common HVAC issue that can stem from multiple components failing safety checks or sensing problems. This article explains the most likely causes, step-by-step troubleshooting, and safe repair options so homeowners can decide whether to DIY or call a professional. Prioritize safety when working with gas, electricity, or combustion appliances.

Symptom	Possible Causes	Quick Fix
Ignites briefly then goes out	Dirty flame sensor, bad gas valve, weak ignition	Clean sensor, test valve, replace faulty parts
Attempts to light repeatedly	Pressure switch/venting problem, blocked condensate	Clear vent/condensate, check inducer motor
Shows error codes	Control board or safety switch	Record code, consult manual or tech

How Furnaces Start And Why They Shut Off Quickly

Modern gas furnaces follow a sequence: thermostat calls for heat, inducer motor runs, pressure switch verifies venting, igniter lights gas, flame sensor confirms combustion, and blower runs. If any safety check fails, the control board shuts the gas off. The furnace lights for 5 seconds then shuts off when ignition occurs but a downstream safety sensor or verification fails shortly after ignition.

Top Causes When A Furnace Lights For 5 Seconds Then Shuts Off

Dirty Or Failing Flame Sensor

The flame sensor is a small metal rod inside the burner assembly. Its job is to detect that a stable flame is present. A thin layer of carbon or oxidation can prevent proper detection. When the control board doesn’t see flame, it shuts the gas valve after a few seconds to prevent unburned gas buildup. This is the most common cause when the furnace lights then self-extinguishes.

Faulty Igniter Or Inconsistent Ignition

If the igniter is weak or intermittent, the burner may light but not sustain a stable flame across all burners. That unstable flame can trigger the control to shut down as the flame sensor or manifold pressure fluctuates. Symptoms include flickering flame or longer cycles of repeated attempts.

Gas Valve Or Gas Supply Issues

A malfunctioning gas valve may open momentarily and then close if it detects abnormal readings or electrical faults. Low gas pressure from the utility or a partially closed gas shutoff can create similar symptoms. Unexpected closures protect the home from unsafe gas conditions.

Pressure Switch Or Inducer Motor Problems

The inducer motor creates draft before ignition. The pressure switch ensures the inducer is producing the correct vacuum. If the inducer is weak or the pressure tubing is clogged, the pressure switch may open after ignition, causing the furnace to shut off. Blocked venting, a dirty inducer, or damaged tubing commonly contribute.

Blocked Venting Or Flue Problems

Blocked or restricted venting prevents combustion byproducts from exhausting. This leads to fluctuating pressures and safety trip conditions. Even partial obstructions from nests, debris, or collapsed PVC can cause the furnace to light briefly then shut down when sensors detect poor exhaust conditions.

Limit Switch Or Overheat Protection

The limit switch detects excessive heat in the heat exchanger. A stuck or faulty limit can trip and shut burners off quickly. Restrictive airflow from a dirty filter or closed vents can warm the exchanger rapidly, triggering a shutdown after brief ignition.

Control Board Or Wiring Faults

Intermittent electrical faults, loose connections, or a failing control board can interrupt the ignition sequence. If the control board loses the flame signal or misinterprets sensor inputs, it will cut gas supply as a safety measure. Error LED codes on the furnace can point to control issues.

Initial Safety Steps Before Troubleshooting

Turn off power to the furnace at the service switch and shut off the gas if a leak or strong odor is present. Use caution when opening panels; some components remain hot. If uncertain about gas or electrical work, contact a licensed HVAC technician to avoid danger.

Step-By-Step Troubleshooting Guide

1. Observe And Record Behavior

Note how long the furnace runs, any blinking error code on the furnace board, auditory cues like repeated clicks, and whether the blower runs. Recording LED blink patterns or error messages helps identify the failing component quickly.

2. Check The Air Filter And Airflow

A clogged air filter reduces airflow, causing heat buildup and limit trips. Replace or clean the filter before further testing. Verify supply registers are open and that the return is not blocked.

3. Inspect Vents And Flue

Visually check the outside vent or chimney for obstructions like nests, snow, or debris. For PVC exhaust, ensure there are no collapsed sections. Clear any visible blockages and test the furnace again.

4. Clean The Flame Sensor

Turn off power and gas. Remove the flame sensor screw and gently clean the rod with fine-grit sandpaper or steel wool until shiny. Reinstall and retest. Cleaning often resolves brief ignition shutdowns. Do not bend or over-sand the sensor.

5. Inspect The Igniter

Visual inspection can reveal cracks or glazing on hot surface igniters or damaged spark electrodes. For hot surface igniters, a multimeter can test continuity; no continuity means replacement. For spark ignitions, listen for a strong, consistent spark.

6. Test The Pressure Switch And Inducer

Listen for the inducer motor running before ignition. If it stalls or is noisy, it may be weak. Inspect pressure tubing for cracks or blockages and replace if damaged. Some pressure switches can be tested with a multimeter when the inducer runs; consult the furnace manual for expected readings.

7. Check Gas Supply And Valve Operation

Verify other gas appliances work to rule out utility supply issues. Inspect the furnace gas valve wiring for loose connections. Valve voltage measurements require care; if not comfortable, have a technician test manifold pressure and valve function.

8. Read Error Codes And Diagnostic LEDs

Most furnaces provide diagnostic blink codes. Count flashes and compare with the manual to identify suspected components. Record codes and consult the manufacturer’s service sheet for targeted troubleshooting steps.

When To Replace Parts Versus Clean Or Adjust

Cleaning a flame sensor or replacing a clogged filter is low-cost and often successful. Igniters, pressure switches, and gas valves require testing and may need replacement when faulty. Control boards and complex gas valves should be replaced by qualified technicians to ensure correct wiring and safety compliance.

DIY Repairs Vs. Professional Service

Homeowners can safely perform non-invasive tasks like changing filters, clearing vents, and cleaning the flame sensor. Electrical testing, gas valve replacement, and control board work involve risk and typically require a licensed HVAC technician. For any gas smell, immediate professional assistance is required.

Estimated Costs For Common Repairs

Repair	Typical Cost Range (Parts + Labor)
Clean Flame Sensor	$40–$120
Igniter Replacement	$150–$400
Pressure Switch/Inducer Repair	$200–$600
Gas Valve Replacement	$300–$900
Control Board Replacement	$250–$800

Preventive Maintenance To Avoid Short Cycling

• Replace air filters every 1–3 months depending on use and filter MERV rating.
• Schedule annual professional tune-ups to inspect burners, sensors, and gas connections.
• Keep vents and exhaust terminations clear of obstructions, snow, and debris.
• Ensure proper combustion air supply in tightly sealed homes or where exhausts are near intakes.

Safety Signs That Require Immediate Attention

• Gas Smell: Evacuate and call the gas company or 911 immediately.
• Repeated Short Cycling: Can indicate a dangerous condition; stop using the furnace and call a pro.
• Carbon Monoxide Alarm Activation: Leave the home immediately and contact emergency services.

How To Talk To An HVAC Technician

When calling a technician, provide the furnace make and model, describe that the furnace lights for 5 seconds then shuts off, report any error code or LED blink pattern, note recent maintenance, and mention any odd odors or noises. This information speeds diagnosis and helps the technician bring the right parts.

Summary Of Troubleshooting Priority

1. Replace or clean the air filter and inspect airflow.
2. Clean the flame sensor.
3. Inspect igniter and inducer motor function.
4. Check venting and pressure switch tubing.
5. Record and report error codes for control board diagnostics.
6. Call a licensed technician for gas valve, control board, or complex electrical repairs.

Addressing the most common causes in order—airflow, flame sensing, ignition, and venting—resolves the majority of issues where the furnace lights for 5 seconds then shuts off. Safe handling of gas and electrical components is essential; when in doubt, consult a licensed HVAC professional to ensure reliable and code-compliant repairs.