If your furnace keeps trying to ignite but never starts properly, or if the burners light briefly and then go out, there is usually a problem somewhere in the ignition or safety sequence.
A gas furnace has to complete several steps before it can begin heating your home. If one of those steps fails, the furnace may stop, wait, and then try again. After several unsuccessful attempts, some furnaces will go into lockout and stop trying altogether.
A dirty flame sensor can cause repeated ignition attempts
One of the most common causes is a dirty or failing flame sensor. The flame sensor confirms that the burners actually lit after the gas valve opened. If the sensor does not detect a proper flame, the furnace shuts off the gas supply for safety. The furnace may then attempt to ignite again. You may hear the burners light normally, run for only a few seconds, shut off, and then repeat the entire startup sequence.
The igniter may be failing
Modern furnaces commonly use an electronic igniter to light the burners. If the igniter is weak, cracked, damaged or failing, the gas may not ignite properly. You may hear the furnace begin its startup sequence, but the burners never light. The control board may then stop the attempt and try again after a short delay. A failing igniter can sometimes work intermittently, which can make the problem seem unpredictable.
The burners may not be lighting properly
Even if the igniter is working, the burners themselves may not ignite correctly. Dirt, rust, debris or other contamination around the burner assembly can interfere with ignition. In some cases, one burner may light while the flame does not travel properly across the rest of the burner assembly. This can cause the furnace to shut down and attempt another ignition cycle. Burner problems should be professionally inspected because they are part of the furnace’s combustion system.
The gas valve may not be opening correctly
The furnace cannot ignite if it is not receiving the proper flow of gas. A problem with the gas valve or another part of the fuel supply system can prevent the burners from lighting. The furnace may still go through its normal startup sequence and repeatedly attempt ignition. Gas valve problems require professional diagnosis and repair.
A pressure switch problem can interrupt ignition
Before the burners light, many furnaces verify that the venting system is working properly. The inducer motor starts first, and a pressure switch confirms that the furnace is able to safely move combustion gases out of the home. If the pressure switch does not close when expected, the furnace may stop the ignition sequence. Possible causes can include:
A blocked vent or intake pipe
A problem with the inducer motor
Damaged pressure-switch tubing
A clogged condensate drain
A faulty pressure switch
Other venting problems
If the condition changes temporarily, the furnace may keep trying to start.
A clogged condensate drain can stop a high-efficiency furnace
High-efficiency furnaces produce condensation as part of normal operation. That water has to drain properly. If the condensate drain, trap or tubing becomes clogged, the furnace may detect an unsafe condition and interrupt the startup sequence. The furnace may then attempt to ignite again after the control board resets. This is one reason ignition problems in high-efficiency furnaces are not always caused by the igniter itself.
The furnace control board may be detecting a fault
The control board manages the furnace startup sequence. It monitors safety switches, ignition, flame detection and other components. If the board does not receive the signals it expects, it may stop the ignition attempt and try again. Some furnaces will display a flashing diagnostic code when this happens. If your furnace has a small viewing window, you may be able to see an LED flashing inside the cabinet. The number or pattern of flashes can help identify what part of the startup sequence is failing.
What does a normal furnace ignition sequence sound like?
A normal gas furnace startup usually follows a recognizable pattern.
You may hear:
The inducer motor start
The igniter activate
The gas valve open
The burners ignite
The blower start after the furnace warms up
If the furnace repeatedly gets partway through this sequence and then starts over, the point where it stops can provide an important clue. For example, if the burners ignite but immediately shut off, the flame sensor may be involved. If the burners never ignite at all, the problem may be related to the igniter, gas supply, pressure switch or another component.
Why does my furnace ignite and then immediately go out?
If the burners light and then shut off after only a few seconds, the flame sensor is one of the first components that should be checked. But it is not the only possible cause. Poor grounding, wiring problems, burner contamination, gas pressure problems or control-board issues can sometimes produce similar symptoms. That is why diagnosing the complete ignition sequence is more reliable than simply replacing parts based on one symptom.
Will the furnace eventually stop trying to ignite?
Often, yes.
Most modern furnaces limit the number of unsuccessful ignition attempts. If the furnace cannot establish and verify a proper flame after several tries, it may enter lockout. The system may remain off for a period of time or until it is reset. Lockout is a safety response. It prevents the furnace from endlessly repeating an unsuccessful ignition sequence.
Should you keep resetting the furnace?
Repeatedly resetting a furnace is not a good way to handle an ignition problem. A reset may allow the furnace to try again, but it does not correct the reason the ignition sequence failed. If the furnace repeatedly has to be reset before it will run, the underlying problem should be diagnosed.
When should you call for furnace repair?
You should have the furnace inspected if:
It repeatedly tries to ignite but cannot start
The burners light and then immediately go out
You hear repeated clicking without ignition
The furnace goes into lockout
The furnace displays the same diagnostic code repeatedly
You smell gas
The problem keeps returning after the furnace is reset
The furnace is no longer heating your home reliably
Ignition problems can involve several different components, so proper testing can prevent unnecessary parts from being replaced.
Need help with a furnace that keeps trying to ignite?
If your furnace repeatedly attempts to ignite, the problem could involve the flame sensor, igniter, burners, pressure switch, gas valve, venting system or another safety component.
Roberts Brothers Comfort can diagnose the furnace startup sequence, identify why ignition keeps failing and make the necessary repairs. We provide expert furnace repair service throughout Ohio’s Miami Valley region including Casstown, Covington, Fletcher, Piqua, Rosewood, Saint Paris, Sidney, Tipp City, Troy, Urbana, West Milton and all the surrounding areas.
