Thermostat displays E13 and may indicate 'Fan control error' or 'G output fault'; blower does not respond correctly.

Safety first: Turn off power to the indoor unit before accessing the blower compartment or control board.

Steps:

• Check G wiring: At the thermostat and indoor control board, verify the G wire is securely connected and not broken.
• Manual fan test: With power restored, set the thermostat fan mode to 'On'. Listen for the blower. If the E13 code appears and the fan does not run, measure 24VAC between G and C at the indoor board.
• Bypass test (advanced): With power off, temporarily jumper R to G at the indoor board. Restore power; if the blower runs, the board and motor are likely good and the thermostat or G wire is suspect.
• Inspect fan relay: On the indoor board, listen for a relay click when G is energized. If no click and no blower, the relay or board may be faulty.
• If thermostat G output is dead: Replace the thermostat. If G output is present but blower does not run, have a technician test and replace the indoor control board or blower relay as needed.