An automotive breakout box is an instrument, housed as the name suggests in a sturdy box or case, that enables an electrical connector in a vehicle to be separated or "broken out" into its component connectors. These connectors can then be tested for communication problems. A breakout box normally attaches to the main ECM harness connector. When the box is in place it gives electrical access to sensors, actuators, circuits, and other elements controlled by the ECM in question. The electrical connections in the box are labelled to match those in the ECM connector.

Breakout boxes are sometimes provided by car manufacturers to enable service engineers to test electric connectors in modern vehicles which are typically densely packed, environmentally sealed, and difficult to gain access to without causing damage. Universal breakout boxes can also be purchased for vehicle diagnosis, and typically cost between $250 and $900, depending on their capability. They require specialist knowledge for effective use.