Lennox Air Conditioner Error Code E345

E345 indicates that the 'O' relay on the outdoor control board has failed or is not functioning correctly. The fault covers three possible conditions: the relay coil did not energize, the relay contacts did not close, or the board's confirmation circuit is not sensing relay state properly. This relay is involved in reversing valve control, which governs heating and cooling mode switching on heat pump systems. The outdoor control board will need to be inspected and most likely replaced to resolve the error.

Operating the system with an active E345 fault is not recommended. A failed 'O' relay can prevent the reversing valve from switching correctly, which may cause the system to run improperly in the wrong mode or not operate at all. Continuing to run the unit in a fault condition can place additional stress on other components. Shut the system down and have it serviced promptly.

A power reset may temporarily clear the E345 error code from the display, but it will not resolve the underlying hardware fault in the 'O' relay circuit. If the relay or control board is genuinely failed, the code will return once the system attempts to operate and the confirmation circuit detects the fault again. A reset should only be used as part of a diagnostic process, not as a long-term solution.

Diagnosing and repairing E345 requires working inside the outdoor unit with live electrical components, which carries serious safety risks. A qualified HVAC technician should perform this repair, as it involves evaluating the outdoor control board and its relay circuitry. While checking external wiring connections is a reasonable first step for experienced DIYers, replacing the outdoor control board should be handled by a professional.

Repair costs for E345 typically depend on whether wiring repairs are sufficient or if the outdoor control board requires replacement. Control board replacements for Lennox outdoor units generally range from $300 to $700 or more including parts and labor, depending on the specific model and local labor rates. A technician visit to diagnose the fault may carry its own service call fee on top of repair costs.