Thermostat shows E280 indicating internal fan control or G output fault; fan may not run when commanded.

Safety warning: Turn off power to the furnace/air handler before working on low-voltage wiring.

• 1. Check indoor unit fuse:
  • Inspect and replace the low-voltage fuse on the Lennox control board if blown.
• 2. Inspect G wiring:
  • Verify the G wire is securely connected at both thermostat and control board.
  • Look for shorts to ground or other conductors.
• 3. Test fan operation from unit:
  • At the control board, jumper R to G briefly (with power on) to see if the blower runs.
  • If the blower runs, the issue is likely in the thermostat or its wiring.
• 4. Replace thermostat if G output is failed:
  • If wiring is good and the blower works when jumpered, replace the Lennox thermostat.