Quick answer
A furnace lockout lasts until the board resets itself or you cycle the power. A soft lockout, set after 3 to 5 failed ignition attempts, clears automatically in roughly 1 to 3 hours depending on the manufacturer. A hard lockout stays until power is switched off for 30 seconds or more. Read the flash code first, reset once only, then call for service.
How long a furnace lockout lasts depends on which kind of lockout the control board has set. A lockout is the board refusing to try again after a safety check failed, and boards have two versions of it: one with a timer and one without. Knowing which you have tells you whether heat will come back by itself tonight or not until someone intervenes.
Soft lockout: the board retries in about 1 to 3 hours
A soft lockout clears itself. When the burners fail to light, or light and fail to prove flame, the board tries again, usually 3 to 5 times with a purge between attempts. Then it stops and starts a timer. One hour is the most common setting and some manufacturers use three; Carrier-family boards, for example, hold an ignition lockout for three hours.
When the timer runs out, the board runs the full start-up sequence again. If the fault was marginal, such as a flame sensor right at the edge of its signal or a gust that upset the pressure switch, the furnace may light and run. If a part has failed outright, the furnace repeats the same failed trials and starts the timer over. A house on that cycle gets a few minutes of heat, or none, every hour or three.
Hard lockout: it lasts until the power is cycled
A hard lockout has no timer. The board sets it for faults it treats as unsafe to retry, which on most furnaces means flame sensed when the gas valve should be closed, a gas valve relay fault or an internal board failure. Some boards add repeated limit-switch trips to that list. It holds until 120-volt power is removed and restored.
Turn the furnace service switch off for at least 30 seconds, then back on, and raise the thermostat. If the fault is still present, the lockout returns on the next attempt, often before the first cycle finishes. A tripped flame rollout switch is separate again: it has its own manual reset button, and a power cycle will not clear it.
Read the flash code before you cut the power, then reset once only
The board tells you why it locked out, and on many furnaces that information is erased when power is removed. Look through the small sight glass in the lower furnace door before you touch the switch. On some boards the code is a simple count of flashes; on others it is a two-digit code of short and long flashes. Write it down or film it.
The legend is printed on a label inside the door or on the back of the upper panel. Codes vary by brand and model, and the label on your furnace wins over any list online. What a flashing furnace light means explains how to read the common patterns.
One power cycle is a fair test. It clears a lockout caused by a one-off event such as a power flicker or a wind gust, and it costs nothing. More than one is a risk. Every ignition trial opens the gas valve for several seconds, and a furnace with a weak igniter or dirty burners may not light that gas right away.
Reset it again and again and the furnace can light late, with a thump as the unburned gas ignites all at once. That is delayed ignition, and it damages burner boxes and heat exchangers. Repeated resets also wipe the code history a technician would have used. The step-by-step procedure is in how to reset a furnace.
The faults behind most furnace lockouts, and what they cost
- Flame sensor not proving flame: the burners light and drop out seconds later. Cleaning or replacement runs $150–$300.
- Igniter not lighting the gas: a cracked or weak hot-surface igniter, $200–$450.
- Pressure switch not closing: a blocked vent, a plugged condensate drain or a failed switch, $250–$500 for the switch itself.
- Inducer motor failing: $400–$900.
- Limit switch opening: restricted airflow from a clogged filter, closed registers or a blower fault.
- Flame rollout: a blocked heat exchanger or vent. Do not press the reset; see why a rollout switch keeps tripping.
Is it safe to wait out a lockout overnight?
- It can wait until morning: a soft lockout, no gas smell, no burning smell, and a house holding above about 15 °C.
- Call now: a lingering gas smell (leave the house and call Enbridge Gas from outside), a tripped rollout switch, scorch marks or soot at the furnace, a carbon monoxide alarm sounding, or anyone vulnerable in a house that is losing heat fast.
- Call the same day: the lockout came back after one reset.
A lockout diagnosis is a $90–$150 visit that is credited to the repair when you approve it, and the written price comes before any part is replaced. ZK Mechanical answers (647) 801-1252 around the clock with no after-hours surcharge; see furnace repair or book a visit. More lockout causes are explained one by one in the heating answers hub.