Using a test light to check for circuit voltage is a tried-and-true diagnostic method. But if you probe a computerized circuit with your old incandescent test light, you can fry some sensitive and expensive electronics. Since almost every circuit in a late-model car is controlled by a computer, it's best to say goodbye to your old test light and invest in a computer-safe test light. These don't pull any power from the tested circuit. Instead, they plug into the cigarette lighter. A red LED means the circuit has power. A green LED means the circuit is connected to ground.