Indoor blower runs continuously even when thermostat is not calling for heat or fan.

Safety: Turn off power before accessing the control board or thermostat wiring.

Steps:

• Check thermostat fan setting: Ensure it is set to AUTO, not ON.
• Disconnect G wire test: At the furnace control board, remove the G wire (fan call) from the thermostat. Restore power. If the blower stops, the thermostat or wiring is causing the fan call.
• Inspect limit switch: If the blower runs but burners do not, the high limit may be open. Check for code 33 or similar on the board.
• Inspect control board: Look for signs of a stuck or burned blower relay.

When to call a technician: If the blower continues to run with the G wire removed, the control board or limit circuit likely needs professional replacement.