Lennox Error Code E240 indicates Low flame current (run mode). Start by checking the most common and safe items identified in the troubleshooting section. If the code returns after those checks, professional diagnosis is recommended.
Easy
Low flame current (run mode)
Unknown
Low flame current detected by flame sensor
Check micro‑amperes of flame sensor; clean or replace sensor; measure voltage of neutral to ground to ensure good unit ground; alert clears after current heat call
Flame sensor, grounding
Call an HVAC technician if the error persists after completing the recommended basic troubleshooting steps or if the system repeatedly locks out.
Disconnect power before inspecting the equipment. Never bypass safety devices or operate the system if you suspect unsafe conditions.
Learn more about the furnace components and operating conditions commonly associated with diagnostic error codes: