A furnace lockout is the control board shutting off the fuel and power because a sensor or limit switch detected a safety problem. Trane says the furnace will not turn back on until the issue that triggered the shutdown is addressed and the furnace is reset. A soft lockout is the first one or two times the controller stops the ignition sequence; a third time makes it a hard lockout that needs a technician.
Manufacturers differ on how long a lockout lasts: Carrier's control resets itself after 3 hours for limit and ignition lockouts, after 1 hour for a control-circuitry lockout, and not at all for a gas-heating lockout, while Lennox uses one-hour soft lockouts. The only thing Trane says to try yourself is changing the air filter. Everything else, including the flame sensor, ignitor, gas pressure and vent, is technician work.
What this covers
- Gas furnaces with electronic ignition and a control board that locks out after failed ignition or a tripped safety switch
- Trane's soft and hard lockout terms and the Carrier and Lennox reset behavior listed below
- Customer-accessible action: the filter, and recording the code
What it does not cover
- ×Standing-pilot furnaces, which do not have this lockout logic
- ×Any reset that bypasses a safety switch
- ×Cleaning the flame sensor or burners, adjusting gas pressure or opening the furnace
Try in this order
0 of 6 completedRecognize a lockout
ReversibleA furnace in lockout tries to start, runs the inducer or clicks, then shuts down and will not retry, or it retries only after a long wait. Trane describes a furnace that starts up several times in a row and then shuts down. Note what the light or display shows.
Know the difference between soft and hard
ReversibleTrane says a soft lockout is the first one or two times the controller stops the ignition sequence, and a third time is a hard lockout. In a hard lockout you need to call for service so a technician can reset the furnace after resolving the cause.
Change the filter, and only the filter
ReversibleTrane says changing the air filter is the only thing you should try yourself. A dirty filter is one of the listed causes along with a dirty or defective flame sensor, a faulty ignitor, fuel-pressure problems, overheating, blocked vent pipes or flue, and a faulty control board.
Try one power reset, if the manual allows it
ReversibleTrane says the owner's manual, if it addresses lockout, may suggest turning power off and on twice in 30 seconds to see whether the furnace resets. Do this once, only if your manual says so. If the furnace locks out again, do not keep trying.
Read the code before it clears
ReversibleCarrier's control resets itself after 3 hours for code 13 (limit circuit lockout) and code 14 (ignition lockout), after 1 hour for code 45 (control circuitry lockout), and does not reset itself at all for code 21 (gas heating lockout). Lennox's table says one-hour soft lockout for E250, E274, E275, E276 and E290 to E292, and lists E270 to E273 as soft lockouts that clear when a heat call finishes successfully. Waiting out the timer restarts the furnace but does not remove the cause.
Give the technician the full picture
ReversibleTell them the brand, model, the code and when it appeared, whether it repeats, and that you changed the filter. Trane lists what a technician does: clean or replace the flame sensor, clean or replace the ignition system, clear burner blockages, check the gas valve, supply and pressure, clear vent pipes, and check the control board.
Evidence ledger
Sources behind this answer
Trane's own guide: soft lockout versus hard lockout, the common causes, that changing the filter is the only thing to try yourself, and what a technician checks.
Trane's own guide: thermostat, power, filter, gas shut-off valve, igniter, flame sensor, pilot light on pre-2010 furnaces and dirty burners, with what to leave to a technician.
Carrier's own guide for the 58CVA and 58CVX. Two-digit status codes flashed on an amber LED, each with its own section.
Lennox's own service literature: the E-code table for the 103130-XX ignition control, including soft lockouts.
Exact questions
Common questions about “Furnace lockout”
What is the difference between a soft lockout and a hard lockout?+
Trane says a soft lockout is the first one or two times the controller stops the ignition sequence. A third time is a hard lockout, which needs a technician to resolve the cause and reset the furnace.
Will my furnace fix itself if I wait?+
Sometimes it retries. Carrier's control resets itself after 3 hours for a limit or ignition lockout, and Lennox has one-hour soft lockouts, but a repeat means the cause is still there. Carrier's code 21 gas-heating lockout does not reset itself at all.
Can I clean the flame sensor myself?+
Trane says not to attempt to clean or replace the flame sensor yourself and to leave it to professionals. It is a common lockout cause, but the work is inside the furnace.
Is a furnace lockout dangerous?+
The lockout itself is the furnace protecting you, so do not defeat it. The danger is the cause: a blocked vent, overheating or a gas problem. Carbon-monoxide alarms, soot or gas odor mean leave and call from outside.
Which codes are lockouts on a Carrier furnace?+
Codes 13, 14, 15, 21 and 45 are lockouts in Carrier's guide. Code 33 is a limit-circuit fault that becomes lockout 13 if the switch stays open longer than 3 minutes.
What is a furnace lockout?+
A safety state after several failed ignition attempts. The furnace stops trying to protect itself.
How does it reset?+
A soft lockout usually clears after a timed period, while a hard lockout needs a power reset. Repeated lockouts point to a fault.
Should I keep resetting?+
No. Repeated resets hide the cause, so call a technician if it returns.