What it means
Improper main ground, OR line voltage below 90 volts. Control will not respond to a heat call until voltage rises above 95 volts.
⚠ Call a profurnace
How it shows: Red LED #1: alternating fast flash; Green LED #2: alternating fast flash
1Likely causes
- Improper main ground
- Line voltage below 90 V
2Safe DIY steps
- Techniciancheck main ground and supply voltage
⚠ 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: electrical supply problems require an electrician or HVAC technician.
Find a local HVAC pro →Other Lennox codes
Alternating slow flash
Watchguard 1 hour: burners failed to ignite or lost flame 5 times during a single heating demand.
Board failure pattern
Circuit board failure or control wired incorrectly.
Red fast flash, green slow flash
Main power polarity reversed. Switch line and neutral.
Red off, green slow flash
Pressure prove switch open, OR blocked inlet/exhaust vent, OR pressure switch closed before the combustion air inducer started.
Red on, green slow flash
Rollout switch open, OR 12-pin connector improperly attached.
Red slow flash, green fast flash
Low flame signal: measures below 1.5 microamps. Replace flame sense rod.