If you are someone that has very limited knowledge on how your car works and operates, you might not even be aware that this plug there. When an error light comes on on your dashboard, your mechanic will plug a little device into this plug to run a diagnostic check to confirm the problem. In almost all cars, this plug is found just below the steering where next to the fuse box. This means that when you plug it in, you will be able to run the wire up along the left side of the dash so that there will not be any messy cables and wires in your way.