What squawk code should be used when communications are lost?

When communications are lost, the appropriate squawk code to use is 7600. This code specifically indicates a loss of communications and is part of the standard system to inform air traffic control and other aircraft of your situation.

Using squawk code 7600 allows other aircraft and air traffic controllers to be aware that you are experiencing communication difficulties, which can help them assess the safety and position of the aircraft. This code is particularly important in maintaining situational awareness and ensuring that appropriate measures can be taken to assist the aircraft experiencing the communication failure.

The other codes have different meanings: 7500 indicates unlawful interference or hijacking, 7700 signals an emergency situation (not specifically related to communications), and 7800 is not a standard squawk code used in aviation.