Many furnace lockouts clear on their own after a set time, or when power or the thermostat call is interrupted briefly. Rheem, for example, says a soft lockout can be reset by turning off the furnace power or the thermostat for 5 to 10 seconds. Do not keep resetting a furnace that locks out again, and never reset a tripped rollout switch without finding the cause.
Lockout timings from the manuals
| Brand / code | What the manual says |
|---|---|
| Carrier 13 / 14 | Limit and ignition lockouts auto-reset after 3 hours |
| Carrier 21 | Gas heating lockout: control will NOT auto-reset |
| Carrier 45 | Control circuitry lockout: auto-resets after 1 hour |
| Trane E2.1 / E2.2 | One-hour lockout after retries or recycles are exceeded |
| Lennox alternating slow flash | Watchguard: 1-hour lockout after repeated ignition failures |
| Rheem 10 | One-hour lockout after repeated faults |
A safe reset
- Note the code on the control board.
- Set the thermostat to off, wait, then back to heat. Or switch the furnace’s service switch off for a few seconds and back on.
- Watch for the furnace to start normally. If the same code returns, stop resetting.
When not to reset
- Rollout switch tripped: Carrier’s manual says the flame rollout switch requires a manual reset and Rheem warns not to reset safety switches without correcting the cause. Rollout means flames came out of the heat exchanger area; this needs a technician.
- You smell gas: leave and call your gas utility.
- Repeated lockouts: the cause is still present.
Safety first: If you smell gas, don’t touch light switches or use a phone inside. Leave the building and call your gas utility from outside. Turn off power to the furnace before opening any panel, and never bypass or jumper a safety switch. Gas, venting, wiring and control-board work is for a licensed HVAC technician.
§FFAQ
Is there a reset button on my furnace?
Some furnaces have a manual reset on a rollout or limit switch. These switches trip for safety reasons; a technician should find out why before resetting them.
How long does a furnace lockout last?
It depends on the brand and code. Carrier lists 3 hours for some lockouts and 1 hour for others; Trane, Lennox and Rheem list one-hour lockouts for repeated ignition faults.
Can I reset by flipping the breaker?
Cutting power briefly is a common reset method, and Rheem lists it for soft lockouts. Note the code first, since it may clear when power is removed.