Lennox Error Code E390 indicates Low GWP relay stuck. 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 GWP relay stuck
Unknown
Low GWP relay stuck in furnace control
Replace or repair control board; fault clears when relay operates normally
Low GWP relay, control board
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: