Furnace Starts, Then Shuts Off? What a Lockout Means for Phoenix Homes

When a gas furnace lights and then shuts off, or tries to start and then quits, a safety control may have shut it down. A flame sensor checks for a burning flame each time the furnace starts, and it shuts the furnace down if it doesn't find one. After repeated failed tries, the system may lock itself out entirely. That locked state is called a lockout.

The cause might be a dirty flame sensor, an igniter that can't light the gas, a gas supply or venting problem, or overheating from a dirty filter. A few of those you can check yourself, but anything inside the furnace's gas, combustion or electrical parts is a job for a technician. If your Phoenix-area furnace has been off since last winter, the first cool night is often when you find out whether it lights and stays lit. If you want a broader look at how home heating systems work, our complete residential heating guide for Phoenix Valley homeowners covers the basics.

What Is a Furnace Lockout?

A lockout is a state where the furnace control stops trying to heat after it detects a problem. One furnace maker's user's manual explains that lockout results when a furnace control detects abnormal conditions. On that model, a lockout shows up as a furnace that won't heat, a circulator blower that may keep running, and a diagnostic code on a status light. The circulator blower is the main fan that pushes air through your ducts, so you may hear air moving even though the furnace isn't heating.

A lockout can come at the end of a string of failed starts. When the thermostat calls for heat and the furnace keeps trying to stay on and failing, the system may eventually lock itself out entirely. A furnace can also end up locked out because of a fault such as a tripped limit switch.

The lockout is not the problem itself. It is the furnace's signal that a problem exists. If that problem is still there after a reset, the furnace can lock out again until the problem is found and fixed.

Key Takeaway: A furnace in lockout has stopped heating because its control detected a problem. If the problem is still there, the furnace can lock out again until someone finds and fixes it.

If You Smell Gas, Leave First

Check for a gas smell before you touch the thermostat, the furnace switch or the breaker. The U.S. Consumer Product Safety Commission's winter safety guidance is direct: "If you smell gas or hear gas leaking, leave your home immediately and contact local gas authorities from outside the home. Do not operate any electronics, such as lights or phones, before leaving."

Every furnace check in the rest of this article is only for a home with no gas smell and no sound of leaking gas.

Common Reasons a Furnace Lights and Then Shuts Off

A furnace that starts and then stops, or tries and quits, can point to one of several parts or conditions. Here they are in plain terms.

A dirty flame sensor

On the most common type, the flame sensor is a thin metal rod that confirms a flame is burning each time the furnace kicks on. If it senses the flame, it tells the control board to keep the heat going. If it doesn't, it shuts the furnace down. Even a thin layer of soot or oxidation on the rod can block the sensor's signal, and one sign is burners that light but shut off after a few seconds.

An igniter that can't light the gas

The igniter starts combustion with heat or a spark that lights the gas entering the burners. If the igniter cannot light the gas, the furnace's safety features shut the furnace down and stop the heating sequence. One warning sign is no orange glow during the ignition sequence. On a system with spark ignition, another is repeated clicking as it tries to light the fuel.

A gas supply problem

The burners need gas to light. Making sure the gas supply is available is one of the basic checks for a gas furnace. Anything beyond that, such as the gas valve inside the furnace, belongs to a technician.

Venting and the pressure switch

A gas furnace has to send its exhaust out through a vent. If you hear the inducer motor, a smaller fan, run but the burners don't light, the cause is likely an ignition or pressure switch issue. A blocked vent can keep the pressure switch from closing, and so can an inducer that isn't running up to speed.

Overheating from a dirty filter or blocked airflow

Your furnace moves air across its heat exchanger, the part that passes heat from the burners into the air for your home. Restricted airflow from a dirty filter can cause the heat exchanger to overheat, triggering a safety shutoff. That shutoff comes from a safety limit switch, which trips when the furnace overheats. Keeping supply and return vents open and unobstructed is another basic airflow check.

Key Takeaway: Burners that light and then cut out within seconds may point to a dirty flame sensor. A furnace that tries but never lights may have an igniter, gas supply or venting problem. A dirty filter can also make the furnace overheat and trip a safety shutoff.

What the Owner's Manual Lets You Check

If the furnace won't try to start at all, a tripped breaker or a thermostat problem may be the reason. Our post on safe checks when a furnace won't turn on walks through those. For a furnace that tries to run and then quits, the owner's manual is your guide to the safe steps. The user's manual mentioned above covers one gas furnace model, but it shows how a manual lays those steps out.

Start with the air filter. That Goodman manual tells owners, "As a homeowner, it is your personal responsibility to keep air filters clean." A clogged filter restricts airflow and can cause the system to shut down, so clean or replace the filter if it looks dirty.

Next, look at airflow around the house and outside. The manual's troubleshooting page says, "Check for blocked return air or supply air grilles throughout your home. Grilles may be blocked by furniture, drapery, clothes, carpeting, etc. Clear any blockage." It also says, "Check intake and vent terminations on high efficiency furnaces to make sure they are not blocked." The terminations are the ends of the intake and exhaust pipes, where they leave the house.

Then check the status light. The same page tells owners to check the sight glass on the blower compartment. If the LED status light is flashing, record the number of flashes, shut the unit down and contact a qualified servicer. The sight glass is a small window in the blower door, and the flash count gives the technician a head start.

For the lockout itself, the manual's lockout reset section lists three ways to reset that model. You can wait for the control to reset itself after a lockout period, interrupt power to the furnace, or interrupt the thermostat signal to the furnace. The manual gives a time for each one. The thermostat signal is the call for heat your thermostat sends to the furnace. Those steps and times are written for that model. Use the reset steps in your own furnace's manual, and if you can't find them, call a technician instead of guessing.

Key Takeaway: In a home with no gas smell, a few checks are up to the owner when a furnace starts and then quits. They are a clean filter, open air grilles, clear intake and vent pipes on high-efficiency models, and the reset steps printed in your own furnace's manual.

Why Should You Stop Resetting a Furnace That Keeps Locking Out?

A reset clears the lockout, but it does not clear the cause. The same manual says: "If the condition which originally caused the lockout still exists, the control will return to lockout. If your furnace frequently locks out, a problem exists which must be corrected. Contact a qualified servicer."

The parts behind a lockout, such as the flame sensor, igniter, pressure switch and limit switch, sit inside the furnace's gas and electrical system. Carrier's troubleshooting guide says, "Do not attempt to diagnose or repair gas, oil, combustion, or electrical components yourself." The igniter, for example, is a delicate and high-voltage component, and oils from bare skin can shorten its life. A certified HVAC technician can inspect the sensor and test the electrical connections to find out whether the sensor is the real problem.

As a family-owned shop in Chandler, we want you to know exactly where the safe checks end and a technician's work begins. If a reset from your manual doesn't hold, leave the furnace off and have it checked. Our heating repair team can track down what tripped the lockout and fix the part behind it.

Key Takeaway: When a lockout keeps coming back after a reset, a problem still needs to be found and corrected. Leave the furnace off and have a technician check it.

Keeping Lockouts From Coming Back

Some lockout causes build up slowly, like soot or oxidation on a flame sensor or a filter that fills with dust. A regular inspection gives a technician the chance to catch them. The CPSC urges a yearly professional inspection of all fuel-burning home heating systems, including furnaces, boilers, fireplaces, wood stoves, water heaters, chimneys, flues and vents.

If your furnace sits unused through the warm months, having it inspected before the next heating season is a practical way to catch a slow-building problem. Between visits, keep an eye on the filter during the heating season. Our winter heating maintenance guide for Chandler homes covers more of the seasonal upkeep that keeps a furnace running.

Frequently Asked Questions

What does it mean when a furnace goes into lockout?

A lockout means the furnace control has stopped trying to heat because it detected a problem. One furnace maker's user's manual says lockout results when a furnace control detects abnormal conditions. On that model, the furnace stops heating, the circulator blower may keep running, and a status light shows a diagnostic code. A lockout can follow repeated ignition attempts that fail. It can also follow a fault such as a tripped limit switch. The lockout is a signal. If the problem behind it is still there, the furnace can lock out again until the problem is found and fixed.

Why does my furnace light and then shut off after a few seconds?

A dirty flame sensor is one common reason. The flame sensor checks for a burning flame each time the furnace starts. If it does not detect a flame, it shuts the furnace down. Even a thin layer of soot or oxidation on the sensor rod can block its signal. When that happens, the burners may light and then cut out after a few seconds. Repeated failures like this may end with the system locking itself out. A certified HVAC technician can inspect the sensor and test its electrical connections.

How do I reset a furnace that is in lockout?

With no gas smell in the home, use the reset steps in the manual for your own furnace. For example, one furnace maker's user's manual lists three ways to reset a lockout on its model. You can wait for the control to reset itself after a lockout period, interrupt power to the furnace, or interrupt the thermostat signal to the furnace. The manual gives a time for each one. Those steps are written for that model. If the lockout keeps coming back, stop resetting and have a technician find the cause.

Should I keep resetting a furnace that keeps locking out?

No. A reset clears the lockout, but it does not correct whatever caused it. One furnace maker's user's manual says, "If the condition which originally caused the lockout still exists, the control will return to lockout. If your furnace frequently locks out, a problem exists which must be corrected. Contact a qualified servicer." We give the same advice for any furnace that keeps locking out: leave it off and have it checked.

Can a dirty air filter make a furnace shut off?

Yes, it can. Restricted airflow from a dirty filter can cause the heat exchanger to overheat, which triggers a safety shutoff. The heat exchanger is the part that passes heat from the burners into the air moving through your home. When a furnace overheats, a safety limit switch can trip and stop the heat. Keeping the filter clean is a homeowner's job, so it is one of the first things to check when a furnace starts and then quits.

What should I do if I smell gas?

The U.S. Consumer Product Safety Commission says, "If you smell gas or hear gas leaking, leave your home immediately and contact local gas authorities from outside the home. Do not operate any electronics, such as lights or phones, before leaving." The furnace checks in this article are only for a home with no gas smell and no sound of leaking gas.

Can I clean the flame sensor or replace the igniter myself?

We recommend leaving both jobs to a technician. Carrier's troubleshooting guide says, "Do not attempt to diagnose or repair gas, oil, combustion, or electrical components yourself." The igniter is a delicate, high-voltage part, and oils from bare skin can shorten its life. A certified HVAC technician can inspect the flame sensor, test its electrical connections, and diagnose whether the sensor is the problem. If there is no gas smell, your part is the filter, the air grilles, the intake and vent pipes on high-efficiency models, and the reset steps in your furnace's manual.

Finding the Cause Behind a Lockout

A furnace that starts and then shuts off has a cause that can be found. Check for a gas smell first, and leave if there is one. With no gas smell, check the filter, the grilles and, on a high-efficiency furnace, the outside intake and vent pipes, and try the reset in your own manual once. If the lockout comes back, that is the point to stop resetting and bring in a technician.

When you want a hand, call us at (480) 689-5167 or reach us through our contact page, and a Viking technician can find out what's behind the lockout.