Furnace Hard Lock Out: Causes and Solutions

You depend on your furnace to keep your house and family warm on even the coldest days. But what if it suddenly shuts down? Or you try to turn on the furnace several times in a row and it starts up but then shuts down. What’s going on?

You may be the victim of a furnace hard lockout, also known as a furnace ignition lockout. Entering furnace lockout mode on a bitterly cold winter day is nothing short of an emergency. Keep reading to learn more about what caused the shutdown and what you can do about it.

It’s important to understand that newer gas furnaces have safety features that include built-in sensors and limit switches. When they detect a safety hazard, they signal the furnace control board (or circuit board) to shut off the power and fuel to your furnace.

This is a furnace lockout. The furnace won’t turn back on until the issue that triggered the shutdown is addressed and the furnace is reset.

A furnace soft lockout is the first one or two times that your furnace controller stops the ignition sequence. If it happens a third time, you enter a hard lockout. If your furnace enters a hard lockout, you will need to call for furnace repair so that a technician can reset your furnace after resolving the cause of the problem. 

Consult your owner’s manual. If it addresses a lockout, it may suggest that you turn the power on-off-on-off twice in 30 seconds to see if that resets the furnace.

There are several common causes of a furnace ignition lockout, including:

• Dirty or defective flame sensor
• Faulty ignitor
• Fuel pressure issues
• Overheating
• Dirty filter
• Blocked vent pipes or flue
• Faulty control board

Changing the air filter is the only thing you should do to try and resolve this issue yourself. Call for HVAC service to find the root cause(s) of the furnace lockout and resolve them so you can use your furnace to warm your house.

This sort of furnace troubleshooting and problem-solving is best left to an HVAC professional. Contact your local Trane HVAC service for help. They know how to fix an ignition lockout on a furnace safely.

The HVAC technician will review the control board error codes on a newer gas furnace, and do old-fashioned troubleshooting on an older furnace. This can include:

• Cleaning or replacing the flame sensor
• Cleaning or replacing the ignition system
• Clearing blockages in burners
• Checking the gas valve, gas supply, and pressure
• Clearing blockages in the vent pipes
• Checking the control board for proper functioning

After a full evaluation and doing any necessary work, the technician will reset your furnace.

Take care of your furnace, and it will take care of you. Follow these tips to avoid a furnace hard lock out. 

The easiest and best way to avoid a furnace lockout and many other furnace problems is to change your furnace filters regularly. This is generally every 30-90 days depending on the quality of the filter, if you have pets, and how often your furnace runs. See our filter maintenance guide for more information.

In addition, for proper airflow, make sure the return air vents aren’t blocked.

The inner workings of your gas furnace, the venting system, and other parts of your forced air system require professional maintenance to keep the furnace running at peak efficiency. Schedule a furnace tuneup each fall. Consider signing up for an HVAC maintenance plan with your preferred HVAC company. They’ll contact you when it’s time for a maintenance call.

Get more furnace maintenance tips.

Opt into Trane Diagnostics for peace of mind. Via the Trane Home App, you give your dealer remote monitoring capabilities. Your Trane smart thermostat sends data in real-time to your dealer, alerting them to any troublesome issues, sometimes before you even realize there’s a problem. 

They can run furnace troubleshooting tests to pinpoint the problem. In some cases, they may be able to solve the problem remotely. In this situation, you would grant them one-time dealer remote access (DRA). 

If they need to schedule an on-site visit, the technician will arrive with the knowledge gleaned from remote access. This enables shorter service calls and ensures they arrive with the proper replacement parts.

Anne Fonda, Content Writer at Trane Technologies

A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by John Kim, Senior Product Manager, Furnaces

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