LED Digital Display and Test Bulb The car electrical tester has a real time voltage display for a quick and accurate reading of the current voltage value. Also, the brightness of the bulb at the end of the circuit probe pen will change with the strength of the voltage so that you can quickly determine the fault of the circuit power supply. Polarity Tester & Frequency Prediction The car fuse tester quickly distinguishes between positive and negative polarity by the red and green colours of the built-in light emitting diodes. When the pen is connected to an AC signal of 5 to 30V, the frequency range can be estimated by the digital screen display. It can be used to measure fuel injector signals, spark plugs, crankshaft sensor signals, etc. Three Working Modes The voltage tester is designed with a switch and a 12V 5W test bulb, so you can easily change the working mode you want. You can choose the LED digital display and the bulb to work at the same time, or you can choose one of them to work to test the voltage and diagnose the faulty battery or fuse circuit accurately.