Step-by-step guide to fixing a gas furnace burner that won't ignite. Check gas supply, igniter, flame sensor, and safety interlocks. 25 years experience.
A gas furnace burner that won’t ignite is usually caused by one of five things: no gas supply, a dirty or misaligned igniter, a faulty flame sensor, a tripped safety interlock, or a locked-out control board. Work through them in that order. Most problems are found in the first two.
I’ve fixed burners in foundries across Southeast Asia, India, the UK, Europe, and North America. The symptoms vary. The causes rarely do.
Safety First — Read This Before You Touch Anything
If you smell gas, stop. Do not try to ignite the burner. Do not flip switches. Open doors and windows. Shut off the manual gas valve. Leave the building. Call a qualified gas engineer.
A gas furnace is not a DIY project if you are not trained. The steps below are for maintenance engineers and operators who understand gas safety. If you are not sure, call a professional.
Step 1: Confirm the Symptom
Not all ignition failures are the same. Define what you see:
No spark, no flame. The igniter does not spark. The burner is silent.
Spark, but no flame. You hear or see the spark, but the burner does not light.
Flame lights, then goes out. The burner fires for 2–5 seconds, then the safety system shuts it off.
No response at all. The control panel is dead. No display, no fan, no spark.
Each points to a different cause. A burner that sparks but won’t light is different from one that lights and then dies.
Step 2: Check the Gas Supply
This is the most common cause. I’ve seen it dozens of times.
Is the manual gas valve open? Someone may have closed it for maintenance and forgotten to reopen it.
Is the gas pressure correct? Check the gauge near the burner. It should be within the range specified by the furnace manufacturer. For natural gas, typical inlet pressure is 7–10 inches water column. For propane, 11–14 inches.
Is the regulator working? A failed regulator can drop pressure below the minimum. The burner won’t light.
Is there condensation in the line? In humid climates, water can block the gas flow. Drain the drip leg.
Has the gas supplier reduced pressure? This happens during peak demand. Monitor pressure over a few days.
I once visited a foundry in Thailand where the burner wouldn’t light every afternoon. Gas pressure was fine in the morning. At 2 p.m., it dropped 20%. The supplier was diverting gas to another customer. The foundry installed a buffer tank. Problem solved.
Step 3: Inspect the Igniter
If gas is reaching the burner, the next suspect is the igniter. Most modern gas furnaces use a spark igniter or a hot surface igniter.
For spark igniters:
Check the electrode gap. It should be 3–4 mm. If it’s too wide, the spark is weak. If it’s too narrow, the spark may not jump.
Clean the electrode. Carbon or dirt insulates the tip. Use fine emery cloth. Do not use a wire brush.
Check the ignition cable. Look for cracks, burns, or loose connections.
Test the ignition transformer. If it’s faulty, no spark. Replace it.
For hot surface igniters:
Look for cracks or breakage. These igniters are fragile. If it’s broken, replace it.
Check resistance. Compare with the manufacturer’s specification. A reading of infinity means it’s failed.
Handle with care. Never touch the igniter element with bare fingers. Oil from your skin causes early failure.
I remember a foundry in Malaysia where the burner wouldn’t light. The operator had replaced the control board, the gas valve, and the flame sensor. Nothing worked. I checked the igniter. The gap was 6 mm. I adjusted it to 3 mm and cleaned the tip. The burner lit on the first try. The repair cost nothing.
Step 4: Clean or Replace the Flame Sensor
If the burner lights but goes out after a few seconds, the flame sensor is the likely culprit.
The flame sensor tells the control board that a flame is present. If it doesn’t detect a flame, the board shuts off the gas within seconds. This is a safety feature. It prevents gas from accumulating.
Symptoms:
- Burner lights, then shuts off after 2–5 seconds.
- The control board locks out.
- You may hear the gas valve click open, then closed.
What to do:
Clean the sensor. Use fine emery cloth or steel wool. Do not use sandpaper.
Check the position. The sensor must be in the flame. If it’s bent or misaligned, it won’t detect heat.
Test the sensor. Use a multimeter to check for continuity or microamps, depending on the type.
Replace if faulty. Flame sensors are inexpensive. Keep a spare.
I’ve seen a flame sensor fail because a spider built a nest inside the burner tube. The sensor couldn’t see the flame. A five-minute clean fixed it. The operator was ready to replace the entire burner.
Step 5: Check Safety Interlocks
Gas furnaces have safety devices that prevent ignition if something is wrong. If any of these trip, the burner won’t light.
Door switch. If the furnace door is open or misaligned, the switch stays open. The burner won’t fire.
Ventilation pressure switch. Ensures the exhaust fan is running. If the fan is off or the pressure switch is faulty, no ignition.
High-temperature limit. If the furnace is too hot, the limit switch opens. The burner won’t fire until it cools.
Emergency stop. Check that it’s not pressed.
I’ve seen more than one furnace fail to light because a door switch was misaligned. The door looked closed. The switch didn’t engage. A five-minute adjustment solved it.
Step 6: Check the Control Board and Lockout
Modern furnaces have a control board that manages ignition, flame sensing, and safety. If the board is in lockout, it won’t try to ignite.
Read the error code. Most boards have an LED that flashes a code. Count the flashes. Look up the code in the manual.
Reset the board. Turn power off, wait 30 seconds, turn power on. Sometimes a transient fault clears.
Check for burn marks. Look for scorched areas on the board. If you find them, replace the board.
Check the fuse. A blown fuse will prevent ignition. Replace with the correct rating.
A foundry in the UK had a burner that wouldn’t light. The control board was in lockout. The error code indicated “flame sensed when no flame present.” The flame sensor was dirty and shorted. Cleaning it and resetting the board solved the problem.
Step 7: Check Air-Fuel Ratio
If the burner sparks and gas flows, but the flame won’t light, the mixture may be too lean or too rich.
Too much air. The mixture is too lean to ignite. Reduce the air damper.
Too little air. The mixture is too rich. It may light, but produce soot. Increase air.
Blocked air inlet. Check for obstructions. Clean the air filter if present.
I use a portable combustion analyser to set the air-fuel ratio. But in an emergency, you can adjust the damper slowly while watching the flame. If it lights and holds, you’re close. Then fine-tune with an analyser.
A Real Case from Vietnam
In 2021, a die casting plant in Vietnam had a 2-tonne gas-fired reverberatory furnace. The burner wouldn’t ignite. The operator had replaced the igniter, the flame sensor, and the gas valve. Still nothing.
I arrived on site. The gas pressure was correct. The igniter sparked. The flame sensor was clean. The control board showed no error.
I checked the air damper. It was fully open. The mixture was too lean to ignite. I closed the damper to 30% and tried again. The burner lit immediately. Then I adjusted it to the correct position using a combustion analyser.
The operator had been adjusting the damper for a different alloy and forgot to reset it. A five-minute fix. But it cost them two days of production.
A Simple Ignition Troubleshooting Checklist
- Smell gas? Stop. Ventilate. Call a professional.
- Is the manual gas valve open?
- Is gas pressure within range?
- Is the igniter sparking? Check gap and clean.
- Is the flame sensor clean and correctly positioned?
- Are all safety interlocks closed?
- Is the control board in lockout? Read the error code.
- Is the air-fuel ratio correct?
- Is the burner nozzle clean?
- Is the exhaust fan running?
Work through these in order. Most problems are found in steps 1–5.
Preventing Ignition Problems
The best fix is prevention. Here’s what I recommend:
- Daily: Check flame colour. Record gas pressure.
- Weekly: Clean igniter and flame sensor. Check for soot.
- Monthly: Test safety interlocks. Check gas train for leaks.
- Quarterly: Clean burner nozzle. Check air damper linkage.
- Annually: Full burner overhaul. Replace worn parts.
A foundry in India started a weekly ignition check. Their unplanned burner downtime dropped by 70% in one year. The cost was almost nothing.
When to Call a Professional
If you are not trained in gas safety, do not attempt repairs on the gas train, control board, or ignition system. Call a qualified engineer.
Some tasks are safe for maintenance staff: cleaning a flame sensor, checking a pressure gauge, resetting a breaker. Others are not. Know the difference.
Final Thoughts
A burner that won’t ignite is not a mystery. It’s a symptom. Find the cause, fix it, and the flame will return.
In my experience, 80% of ignition failures come from three things: low gas pressure, a dirty flame sensor, and a misaligned igniter. Check those first. Then look deeper.
And remember: the best troubleshooting is prevention. A good maintenance checklist — like the one I’ve written about — will prevent most of these problems before they happen.
Internal links: Furnace Maintenance Checklist, Why Does My Aluminium Furnace Keep Shutting Down?
About the Author: Hu Shenyue is a veteran aluminium melting furnace expert with 25 years of hands-on experience in China, Southeast Asia, India, the UK, and Europe. He writes practical, experience-based content on furnace selection, maintenance, and optimisation at SmeltPro.
