The system will not run in cooling mode, an E9 code is present on the communicating thermostat or board, and other functions may be locked out.

Safety first: Turn off power to indoor and outdoor units before changing wiring. Configuration steps are low-voltage, but miswiring can damage boards.

Steps:

• 1. Verify equipment match: Confirm the Armstrong Air outdoor unit model matches the indoor furnace/air handler and thermostat type recommended by the manufacturer.
• 2. Check communication wiring: Inspect the low-voltage cable between the thermostat, indoor unit, and outdoor unit. Ensure conductors are landed on the correct terminals per the Armstrong Air wiring diagram.
• 3. Enter installer setup: On the communicating thermostat, access the installer or advanced setup menu. Verify that the correct indoor and outdoor models, tonnage, and stages are selected.
• 4. Reset configuration: If available, perform a system re-scan or auto-detect of connected equipment. Save and exit.
• 5. Power cycle: Turn power off to all components for 5 minutes, then restore. Check if E9 clears.
• 6. If E9 remains: The indoor or outdoor control board may have corrupted firmware or hardware failure. Contact an Armstrong Air dealer or technician for board replacement and reconfiguration.