Open the distribution box and check for dust and debris accumulation. Look for any signs of burnt or damaged wiring. This component receives partially treated liquid waste, known as effluent, from the septic tank's outlet pipe. If it's not working properly, you could face serious issues like backups or flooding. Knowing how to inspect and test a septic distribution box can help catch problems early and. A septic distribution box (D-box) is a concrete or plastic junction that evenly distributes wastewater from your septic tank to all drainfield lateral lines. When it fails, symptoms include uneven wet spots in the yard, slow indoor drains, and sewage odors. Grab your flashlight and tools—we're going in! 1.