Status codeRed slow flash, green onLennox
What it means
Primary or secondary limit switch open. Limit must close within 3 minutes or the unit goes into 1-hour Watchguard.
Call a profurnace
How it shows: Red LED #1: slow flash; Green LED #2: on
§1Likely causes
- Limit switch open (overheating condition)
§2Safe DIY steps
- Turn off power and check the air filter; replace if dirty
- Techniciandiagnose why the limit opened
Safety firstIf you smell gas, do not touch any electrical switch or use a phone in the building; leave immediately and call your gas supplier from outside. Turn off electrical power at the furnace switch or breaker before opening any panel, and never bypass or jumper a limit, rollout or pressure switch. Gas valve, burner, venting, wiring and control-board work should be done by a licensed HVAC technician.
§3When to call a pro
Call a licensed HVAC technician: if the code repeats or the unit enters Watchguard.
Find a local HVAC pro →§4Other Lennox codes
Alternating fast flashLennox
Improper main ground, OR line voltage below 90 volts. Control will not respond to a heat call until voltage rises above 95 volts.
Alternating slow flashLennoxWatchguard 1 hour: burners failed to ignite or lost flame 5 times during a single heating demand.
Board failure patternLennoxCircuit board failure or control wired incorrectly.
Red fast flash, green slow flashLennoxMain power polarity reversed. Switch line and neutral.
Red off, green slow flashLennoxPressure prove switch open, OR blocked inlet/exhaust vent, OR pressure switch closed before the combustion air inducer started.
Red on, green slow flashLennoxRollout switch open, OR 12-pin connector improperly attached.