Thermostat shows E05 and fan will not run in Auto or On mode.

Safety first: Turn off power to the HVAC system at the breaker before accessing wiring.

• Step 1 – Check thermostat G connection: Remove the thermostat and verify the G wire is fully inserted and clamped in the G terminal. Tighten if loose.
• Step 2 – Inspect at control board: At the furnace/air handler, confirm the G wire is secure on the G terminal of the control board.
• Step 3 – Test fan with jumper: With power on, briefly jumper R to G at the control board. If the blower fan starts, the fan and board are good and the issue is in the thermostat or wiring.
• Step 4 – Continuity check: With power off and G disconnected at both ends, test continuity of the G conductor. Use a spare conductor if the original is open.
• Step 5 – Inspect fuse: Check the low‑voltage fuse on the control board and replace if blown, but only after confirming there is no short in the thermostat wiring.
• Step 6 – Replace thermostat: If wiring and board are good, the G relay on the Geo thermostat PCB may be failed. Replace the thermostat head unit.