Lennox Air Conditioner Error Code E120

E120 indicates that the outdoor unit did not respond to the system within the expected communication window. It is a communication timing fault between the indoor controller and the outdoor unit. In many cases the alarm clears on its own once the outdoor unit re-establishes contact with the system.

Start by checking all low-voltage wiring connections at both the indoor and outdoor units, making sure every terminal is tight and free of corrosion. Also confirm that the outdoor unit has stable power by checking its dedicated breaker and disconnect. If the wiring looks intact and power is good, allow the system a few minutes to see if communication is restored and the code clears on its own.

If the issue is simply a loose wiring connection, repair costs are typically minimal, covering only a service call fee which can range from roughly 75 to 150 dollars. If a damaged communication wire needs to be replaced, costs may range from 100 to 300 dollars depending on the length of the run and labor involved. A faulty outdoor unit control board is the most expensive scenario, with parts and labor potentially ranging from 300 to 700 dollars or more.

The E120 alarm is designed to clear on its own once the outdoor unit responds to the system, so a manual reset is not always necessary. If the code persists after the outdoor unit comes back online, inspect all wiring connections before attempting a hard reset. Repeatedly resetting without addressing the root cause may allow the fault to return.

E120 is a communication delay fault rather than a safety-critical alarm in most cases, but the system may not operate properly if the outdoor unit remains unresponsive. It is best to investigate the cause promptly rather than continuing to run the system in a fault state. If the system is not cooling and the code persists, shut the unit down and have the wiring inspected.