1Double Hole Design: Spark plug tester has two spark plug socket, you could compare and test to make an accurate judgment at the same time. 2Time Saving and Efficient: The working principle of the car spark plug tester is to test the performance of the spark plug by simulating the work of the engine. 3Protective Cover: The protective cover can prevent you from the risk of electric shock during the test,which effectively guarantees your safety during the operation. 4Practical Tool: Adjustable frequency of 500rpm?6000rpm. If you suspect that any of the spark plugs are not working properly, you can conduct comparative tests to make an accurate judgment. 5Application: Easy to operate, plug and play. Built in indicator light for more intuitive display of status. Suitable for spark plugs of most 12V gasoline cars.