If your furnace tries to start several times and then stops working altogether, it may have gone into lockout.
Furnace lockout is a safety feature. When the furnace detects that something is preventing it from starting or operating correctly, the control board may stop additional ignition attempts. Depending on the furnace, it may try again after a certain amount of time, or it may remain locked out until the problem is corrected and the system is reset. Lockout itself is not usually the problem. It is the furnace responding to another problem.
Why do furnaces have a lockout mode?
A gas furnace follows a specific startup sequence.
It may first start the inducer motor, verify proper venting, activate the igniter, open the gas valve and then confirm that the burners have lit. If one of those steps does not happen correctly, the furnace may shut down and try again. After several unsuccessful attempts, the control board may stop the startup sequence and place the furnace into lockout. This helps prevent the furnace from repeatedly trying to operate when a safety condition has not been satisfied.
A flame sensor problem can cause lockout
One common cause is a dirty or failing flame sensor.
The flame sensor confirms that the burners are producing a flame after the gas valve opens. If the furnace does not receive the proper flame signal, it shuts the gas valve off. The furnace may try to ignite again several times. If it continues failing to detect a flame, the control board may eventually lock the system out. This is why a furnace that lights for a few seconds and then shuts down repeatedly may eventually stop trying altogether.
A bad igniter can prevent the furnace from starting
Modern furnaces commonly use an electronic igniter instead of a standing pilot light.
If the igniter is cracked, worn or failing, the burners may not light. The furnace may make several attempts before entering lockout. You may hear the furnace begin its normal startup sequence but never hear or see the burners ignite. An igniter problem should be properly diagnosed before parts are replaced because other ignition problems can cause similar symptoms.
Problems with the pressure switch can cause lockout
Before ignition, many furnaces verify that the venting system is operating correctly.
The inducer motor creates the airflow needed to safely remove combustion gases, and a pressure switch helps confirm that the proper conditions exist. If the pressure switch does not close when expected, the furnace may not allow ignition to continue.
Possible causes can include:
A blocked vent or intake pipe
A problem with the inducer motor
A clogged condensate drain on a high-efficiency furnace
Damaged pressure tubing
A faulty pressure switch
Other venting or airflow problems
Because these components are part of the furnace’s safety system, repeated pressure-switch errors should be professionally diagnosed.
The furnace may be overheating
A furnace can also shut down because it becomes too hot.
A dirty filter, blocked airflow, blower problem or another restriction can cause excessive heat to build inside the furnace. A safety limit switch may then shut the burners off. If the overheating condition keeps happening, the furnace may eventually stop operating normally or display a fault condition. Check the furnace filter and make sure the supply and return vents are not blocked, but have the system inspected if overheating continues.
Gas supply problems can prevent ignition
A furnace needs a reliable gas supply to ignite and operate properly.
If the gas valve is not opening correctly or there is another fuel-supply problem, the burners may fail to ignite. The furnace may continue trying until it reaches its allowed number of attempts and enters lockout. Gas-related problems should be handled by a qualified technician.
Venting problems can trigger safety shutdowns
Gas furnaces must safely exhaust combustion gases from the home.
If the venting system is blocked or restricted, the furnace may not allow the burner sequence to continue. High-efficiency furnaces can also experience problems related to condensate drainage, especially if a drain line becomes blocked. These safety systems are designed to prevent the furnace from operating under unsafe conditions.
How can you tell if your furnace is in lockout?
The exact symptoms vary by furnace, but common signs include:
The furnace tries to ignite several times and then stops
The burners light briefly and repeatedly go out
The blower or inducer runs but the burners do not stay on
The furnace stops responding even though the thermostat is calling for heat
A diagnostic light on the furnace control board begins flashing a fault code
Many furnaces have a small viewing window where you can see the diagnostic LED. The flashing pattern may correspond to an error code listed on a label inside the furnace panel. That code can give a technician an important clue about what caused the shutdown.
Can you reset a furnace that is in lockout?
In some cases, cycling power to the furnace may reset the control board and allow it to attempt another startup.
But resetting the furnace does not fix the problem that caused the lockout. If the furnace starts normally afterward and continues operating without trouble, the lockout may have resulted from a temporary condition. If it goes into lockout again, repeatedly resetting it is not a good solution. A furnace that continues to lock out is telling you that something in the startup, combustion, airflow or safety system needs attention.
Should you keep resetting a furnace in lockout?
No.
Repeatedly resetting the furnace can hide an underlying problem without correcting it. If the furnace repeatedly fails to ignite, overheats or trips a safety control, the cause should be diagnosed. That is especially important when the problem involves combustion, gas flow or venting.
When should you call for furnace repair?
You should have the furnace inspected if:
It repeatedly goes into lockout
It tries to ignite several times but cannot stay running
The burners light and then quickly shut off
The furnace displays the same diagnostic fault repeatedly
You hear unusual noises during startup
The furnace is overheating
You smell gas, burning or unusual electrical odors
Resetting the furnace only fixes the problem temporarily
A technician can read the furnace fault codes, test the safety controls and identify the reason the system is shutting itself down.
Need help with a furnace that keeps going into lockout?
Furnace lockout is usually a symptom of another problem, such as an ignition failure, flame sensor issue, airflow restriction, venting problem or safety control fault.
Roberts Brothers Comfort technicians can diagnose the reason your furnace is entering lockout and repair the underlying problem so the system can operate normally again. 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.
