High-Efficiency Furnace Main Burner Shuts Off After Initial Ignition: Causes and Fixes

A high-efficiency furnace main burner that shuts off after initial ignition is a common and frustrating issue for homeowners and HVAC technicians. This article explains likely causes, diagnostic steps, safety considerations, and practical fixes to restore reliable operation. The focus is on common induced-draft, direct-vent, and condensing furnaces used in U.S. homes.

Symptom	Likely Causes	Typical Fix
Main burner lights then dies	Flame sensing, gas valve, pressure switch	Clean sensor, test switches, replace parts
Blower runs but no sustained flame	Flame rollout, vent blockage, draft inducer	Clear vent, service inducer motor
Frequent lockouts	Control board, wiring, sensor faults	Diagnostics, replace control or wiring repair

How High-Efficiency Furnaces Work And Why This Problem Occurs

High-efficiency furnaces use sealed combustion, induced-draft blower motors, and condensing heat exchangers to extract more heat from fuel. They rely on multiple interdependent safety and control components, including pressure switches, flame sensors, gas valves, and the control board. When the main burner lights and then shuts off, the system is typically detecting a fault or unsafe condition after initial ignition, causing the control board to stop the gas flow to protect the home and equipment.

Common Causes Of The Burner Shutting Off After Ignition

1. Dirty Or Faulty Flame Sensor

The flame sensor detects the presence of a stable flame. If contaminated by soot, scale, or film, it may fail to sense flame and shut the gas valve quickly. This is one of the most frequent causes when the burner starts then stops within seconds.

2. Pressure Switch Or Venting Issues

High-efficiency furnaces use a pressure switch to confirm the inducer motor is creating proper draft. If a vent is blocked, the pressure switch may open after ignition and de-energize the gas valve, causing the burner to go out.

3. Inducer Motor Or Draft Inducer Failure

If the inducer motor stalls or loses speed after the initial start-up, the system detects insufficient draft and will shut off the burner as a safety measure. Intermittent inducer operation can create the described symptom.

4. Faulty Gas Valve Or Low Gas Pressure

A failing gas valve or inadequate gas pressure can allow ignition but fail to sustain the proper flow. This may cause flame instability and consequent shutdowns. Low utility gas pressure or partially closed valves also cause similar behavior.

5. Limit Switches And Flame Rollout Sensors

Limit switches and rollout sensors detect abnormal temperatures or flame escaping the combustion chamber. If triggered after ignition — possibly due to blocked heat exchangers or restricted flue — they will cut off the burner to prevent hazards.

6. Control Board Or Ignition Module Faults

The main control board or ignition control can fail intermittently, initially allowing ignition and then detecting an error or losing a key signal, resulting in burner shutdown and lockout conditions.

7. Dirty Burners Or Improper Combustion

Accumulated deposits on burners can cause unstable ignition and flame lift, prompting flame-sensing circuits to drop out. Poor combustion can also create soot that fouls sensors quickly.

How To Diagnose The Problem Safely

Diagnosing a furnace that lights then shuts off requires methodical checks and safe practices. Turn off power and gas when inspecting mechanical components. If the homeowner is not comfortable, call a licensed HVAC technician. The following steps outline a technician-style diagnostic flow.

Step 1: Check Error Codes And Control Board LEDs

Most modern furnaces display diagnostic codes via blinking LEDs or error messages. Record the code pattern and consult the manufacturer’s documentation. Error codes often point directly to pressure switch, flame sensor, or inducer faults.

Step 2: Observe The Startup Sequence

Watch the furnace through a complete cycle to note where the sequence fails. Confirm that the inducer runs, the pressure switch closes, the ignition sequence begins, the flame establishes, then whether a shutdown and fault occur. Timing and sequence detail provide critical clues.

Step 3: Inspect And Clean The Flame Sensor

Remove the flame sensor and clean it with a fine emery cloth or specialized sensor cleaner. Reinstall and retest. A cleaned sensor restores flame detection in many cases. Replace the sensor if pitted or heavily corroded.

Step 4: Test The Pressure Switch And Venting

Inspect intake and exhaust terminations for obstruction from birds, debris, or ice. Test pressure switch continuity during startup using a multimeter or manometer to verify proper operation. Replace switched or repair venting if faulty.

Step 5: Measure Gas Pressure And Inspect Gas Valve Operation

Use a manometer to measure inlet and manifold gas pressure during operation. Compare readings to the furnace specification. Inspect control signals to the gas valve; a failing valve or weak solenoids may need replacement.

Step 6: Evaluate The Inducer Motor And Combustion Air Flow

Check inducer motor bearings, capacitor (if present), and electrical connections. Measure inducer RPM or vacuum to ensure it maintains required draft during operation. Replace or service the inducer motor when performance is compromised.

Step 7: Examine Heat Exchanger And Rollout Conditions

Inspect heat exchangers for cracks, heavy soot, or blockages that could cause flame rollout or overheating. Test rollout and limit switches; replace any that trip out or show inconsistent behavior.

Step 8: Check Wiring And Control Board Signals

Inspect wiring harnesses, terminals, and ground continuity. Look for intermittent connectors or damaged insulation. If control board behavior is erratic, document symptoms and consider swapping with a known-good module when appropriate.

Practical Fixes And Maintenance Steps

Routine Cleaning And Maintenance

Regularly clean the flame sensor, burners, and combustion areas. Replace filters and maintain proper airflow. Annual professional tune-ups reduce the chance of intermittent shutdowns by addressing small problems early.

Replace Worn Components

If diagnostics show a failing flame sensor, pressure switch, inducer motor, or gas valve, replace the defective part with OEM-compatible components to ensure proper operation and safety compliance.

Clear And Protect Vent Terminations

Remove obstructions and install protective screens or cages on intake/exhaust terminations to prevent animals, nests, and debris from causing intermittent pressure switch trips.

Adjust Or Test Gas Pressure

Have a qualified technician test and adjust gas pressure to manufacturer specifications. If the utility supply pressure is low, the provider may need to address the issue. Never adjust gas valves without proper tools and training.

Address Control Board Issues Carefully

Control board failures should be handled by technicians. Ensure proper power supply and replace boards only with compatible, manufacturer-approved units. Keep records of error codes and symptoms to streamline repair decisions.

When To Call A Professional

Immediate professional service is recommended if any of these conditions are present: persistent lockouts, gas smell, evidence of carbon monoxide (CO) alarm triggers, visible heat exchanger damage, or uncertain electrical/gas system diagnosis. A licensed technician can perform safe combustion analysis and CO testing.

Safety Considerations And Carbon Monoxide Risks

Furnace shutdowns are safety responses. However, persistent combustion issues increase CO risk. Install CO detectors near sleeping areas and furnace rooms. If a CO alarm activates or a gas odour is detected, evacuate the home and contact emergency services and the gas utility.

Preventive Measures To Avoid Recurrence

• Schedule annual professional maintenance before the heating season.
• Replace air filters every 1–3 months to maintain airflow and heat exchanger performance.
• Keep vent terminations clear and install guards to prevent blockages.
• Document any recurring error codes and repairs for warranty and service records.
• Consider a furnace upgrade if the appliance is beyond recommended service life or experiences frequent major component failures.

Costs And Replacement Considerations

Repair costs vary by component: flame sensor replacement is typically low-cost; inducer motors, pressure switches, and gas valve replacements are moderate; control board or heat exchanger replacement can be expensive. If repair costs approach half the value of a new high-efficiency furnace, replacement may be more cost-effective, considering improved efficiency and warranty coverage.

Key Troubleshooting Checklist For Technicians

1. Record error codes and run a complete start-up observation.
2. Clean flame sensor and burners.
3. Inspect vent terminations and pressure switch operation.
4. Measure gas pressures and verify gas valve operation.
5. Test inducer motor performance and draft strength.
6. Inspect heat exchanger and rollout/limit switches.
7. Check wiring, grounds, and control board signals.

Additional Resources And Manufacturer Guidance

Refer to the furnace manufacturer’s technical manual for model-specific startup sequences, pressure switch diagrams, and error code explanations. Industry resources such as NATE, ACCA, and the U.S. Department of Energy provide guidance on maintenance, combustion efficiency, and safety best practices.

Summary Of Key Points

Most Common Causes: dirty flame sensor, pressure switch/vent blockage, inducer motor faults, gas valve issues, control board errors. Primary Actions: clean the sensor, verify venting and pressure switch operation, test inducer and gas pressures, inspect wiring and control board. Safety: prioritize CO detection and professional service for gas or heat exchanger concerns.

Addressing a main burner that shuts off after ignition requires a systematic approach combining observation, testing, cleaning, and component replacement. Timely maintenance and professional diagnosis help ensure safe, reliable furnace operation and can prevent costly failures and safety hazards.