Thermostat displays E12 and fan will not respond to commands

Safety: Turn off power to the furnace/air handler at the breaker.

• Inspect G wiring: Remove the thermostat and check the G wire for exposed copper or contact with other terminals.
• Isolate circuit: Disconnect the G wire at the thermostat, restore power, and see if E12 clears. If it does, the fault is in the wiring or fan relay coil at the furnace.
• Check furnace board: At the furnace control board, verify the G terminal connection is tight and that the low-voltage harness is not pinched.
• Continuity and short test: With power off, test the G conductor for shorts to other thermostat wires using a multimeter.
• Replace thermostat: If E12 persists with G disconnected, the internal G relay or driver is likely failed and the thermostat should be replaced.