How to Reset a Gas Furnace Safely, and When a Reset Is the Wrong Move

To reset a gas furnace, first write down the blink code on the status light, then turn off the furnace's service switch (it looks like a light switch on or near the furnace) or its circuit breaker, wait 30 seconds, and turn it back on, as Carrier describes. Do it once. If the furnace shuts down again, the fault that stopped it is still there, and the next step is a licensed HVAC technician, not another reset. Never press a reset button on the burner box: that is a flame rollout switch, and Lennox says it trips when flames escape the combustion zone or heat climbs too high.

“Reset” covers a few different things, and only some of them are yours. This page sorts them out. If the furnace won't start at all and you haven't checked the basics yet, start with furnace not turning on, which walks through the full checklist in order.

What a Furnace Reset Actually Does

A modern gas furnace has a control board that runs the startup and watches its safety switches. When something goes wrong, such as burners that won't light or a switch that opens, the board stops the furnace and, after enough failures, locks it out. Cutting the power clears that lockout so the furnace tries again from the start. It doesn't fix anything.

How many tries the furnace gets. Manufacturers build the limit into the control board:

• Rheem: its gas furnace FAQ says the board goes into a soft lockout when no flame is sensed after four ignition trials, or after four flame failures during one call for heat. Turning off the furnace's power, or turning the thermostat off, for 5 to 10 seconds resets it.
• Carrier: the 58SC0B installation manual says the control tries to light the burners four times before going to ignition lockout, and resets automatically after three hours or when power is briefly interrupted.
• Trane: the first one or two times the control stops the ignition sequence are a soft lockout; the third time puts the furnace in a hard lockout, and Trane says a technician then has to reset it after fixing the cause.

So by the time the furnace locks out, it has usually already tried several times on its own. That is why one reset from you is enough to tell you whether the problem was a fluke.

Before You Reset: Read the Status Light

Most furnaces show a blinking light on the control board, often visible through a small window in the lower door. Read it before you touch the power, while the furnace is still showing the fault that stopped it. Watch for a minute and write down the pattern, such as three blinks and a pause, as Trane advises. Trane says red blinks generally signal a problem, while a slow green blink may just mean there is no call for heat.

Codes differ by maker and model. Rheem's are a count of blinks; the Carrier 58SC0B shows a two-digit code, and only the main code is visible through the sight glass. Match yours to the chart on your own furnace, usually on a label inside the door, not a generic list online. Bryant says to write down any codes or blink patterns and give them to your dealer, and that the exact codes depend on the furnace model.

How to Reset a Gas Furnace, Step by Step

1. Check the thermostat first. Carrier says to make sure it's set to HEAT and at least 3 to 5 degrees above the room temperature, and notes dead batteries can keep a thermostat from calling for heat. A blank or frozen thermostat is a different reset; see below.
2. Write down the blink code.
3. Check the filter. Carrier recommends checking the air filter whenever you troubleshoot a furnace, because a clogged one can make it overheat and shut down. If you can't see light through it, replace it before you reset; our filter guide shows how. Otherwise the reset just repeats the same shutdown.
4. Turn the power off. Use the furnace's service switch, which Trane says looks like a light switch on the side of the furnace, on a nearby wall, or on a ceiling joist, or the furnace breaker in the panel.
5. Wait 30 seconds (Carrier), then turn it back on.
6. Set the thermostat to call for heat and watch. Carrier's 58SC0B manual describes the normal order: the inducer (a small draft fan) starts, the hot surface igniter warms up, the gas valve opens and the burners light, and after a short delay the main blower comes on.

If it runs, keep an eye on it for the rest of the day. If it locks out again, stop. Trane says changing the filter is the only thing a homeowner should do to resolve a lockout; the rest belongs to a technician.

If the Breaker Has Tripped

A tripped breaker sits in the middle. Carrier says to flip it to OFF, then back to ON. Reset it once: Bryant says if it trips again, leave it off and call a professional, and Trane calls a second trip an electrical issue. Our breaker keeps tripping page explains why.

The Blower Door Switch

The lower door on most furnaces presses a safety switch. Carrier's 58SC0B manual says the blower access door switch cuts 115-volt power to the control, so a door that isn't seated, often after a filter change, looks exactly like a dead furnace. American Standard also says to make sure the furnace door is closed after changing the filter. Push the door fully closed and fasten it. Carrier's manual says not to tape the switch or let it be bypassed.

Resetting the Thermostat

If the thermostat screen is frozen or blank, Trane's instructions for its own thermostats are to reboot from the menu, or turn off the breaker that powers it, usually the furnace or air handler breaker, for 20 to 30 seconds. For battery models, Trane says to remove the batteries for 2 minutes. Other brands differ; check your thermostat's manual. More fixes are on our thermostat not working page.

Switches You Should Not Reset

The flame rollout switch. This is the small button on the burner box. Lennox says it sits near the burners and stops the fuel when flames escape the combustion zone, and that a manual-reset version needs someone to push the button before the furnace can fire again. Carrier's 58SC0B manual says the rollout switch requires manual reset and not to bypass it, and to correct an inadequate combustion air problem before resetting. Lennox warns that a rollout event can also produce carbon monoxide. Carrier's consumer page describes pressing it to reset, but a trip always has a cause, and finding it means working on combustion air, venting, and burners, so leave it to the technician. Lennox's line: “Multiple trips in a season mean you need a technician, not a reset button.” Our flame rollout page covers the causes.

Limit switches. Bryant says a limit switch shuts off the burners when the furnace overheats, usually from airflow problems like a dirty filter or blocked vents. Some reset themselves once the furnace cools; others have a manual button. Bryant says to never bypass a limit switch and to have a professional find the cause when it keeps tripping. A switch that keeps opening is doing its job; the problem is somewhere else.

Blocked vent and draft switches. Carrier's manual lists the limit, draft safeguard, flame rollout, and blocked vent switches together: if one stays open longer than 3 minutes, the board locks the furnace out for 3 hours. They are venting and combustion safeties, and a lockout from one is a call for the technician.

What Not to Do

• Don't keep resetting. Each lockout means the furnace already tried and failed several times. Bryant says repeated safety shutdowns need professional diagnosis.
• Never reset a manual-reset rollout switch yourself, and never jumper or bypass a rollout, limit, or door switch (Carrier, Bryant, Lennox).
• Don't open the burner compartment. Trane says not to try to fix the furnace yourself, since it's hazardous and could void the warranty.
• Don't reset at all if you smell gas or a CO alarm is sounding. Get out first.

When to Call a Pro

Call a licensed HVAC technician when:

• The furnace locks out again after one reset (Trane).
• The breaker trips a second time (Bryant, Trane).
• The rollout switch has tripped, or a limit switch keeps tripping (Lennox, Bryant).
• Carrier lists banging, grinding, or squealing noises, or burning smells, as reasons to call.
• The furnace keeps starting and then shutting off; Bryant says to have a qualified technician diagnose the cause. See short cycling.

Tell them the blink code, whether the small fan ran, whether the igniter glowed or clicked, and whether the burners lit. If you'll be away while you wait, keep the heat no lower than 55°F, the American Red Cross minimum. If you use an electric space heater in the meantime, the CPSC says to keep it at least 3 feet from anything that can burn, plug it straight into a wall outlet, never a power strip, and never leave it running unattended or on while you sleep.

Prevent It

• Change the filter on schedule. Bryant names a dirty filter among the airflow problems that trip a limit switch, and Carrier says a clogged one can cause overheating shutdowns.
• Keep registers and returns open. Bryant lists blocked vents among the airflow problems that overheat a furnace.
• Book a yearly inspection. The CPSC says to have the heating system professionally inspected and serviced every year.
• Keep CO alarms on every level of the home and outside sleeping areas, as the CPSC recommends.

Frequently Asked Questions

Where is the reset button on a gas furnace?

You reset a gas furnace by switching the power off and on at the service switch or breaker, as Carrier describes. The button some furnaces have on the burner box is the flame rollout switch (Carrier), and it should be reset by a technician who finds why it tripped.

How many times can I reset my furnace?

Once. Rheem and Carrier furnaces try to light four times before locking out, and Trane says a third stopped ignition sequence puts the furnace in a hard lockout that a technician has to reset. If your one reset doesn't hold, call.

How long should I leave the furnace off when resetting it?

Carrier says 30 seconds. Rheem says 5 to 10 seconds of power off, or the thermostat off, clears its soft lockout. Check your furnace's manual for your model.

Will my furnace reset itself?

Some will. Carrier's 58SC0B control resets an ignition or limit lockout automatically after 3 hours. That doesn't mean the fault is gone; a furnace that keeps locking out needs a technician.

The Bottom Line

Check the thermostat and filter, write down the blink code, then switch the power off for 30 seconds and back on, once. Reset a tripped breaker once. Leave the rollout and limit switches, and anything inside the burner compartment, to a licensed HVAC technician. If you smell gas or a CO alarm sounds, don't reset anything; get out and call 911.

Sources

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