Strong Function: This spark test plug is very convenient to understand the operation of the ignition coil, it is the perfect diagnostic tool for accurately identifying a weak spark in minutes. Features: The coil tester diagnostic tool is processed by professional manufacturing, it has high visibility white lettering, read the gap measurement from 0 to 40k. Quick and Fast: In case you are stuck up in no-start conditions, this tester is the quickest way to test your spark plug. Application: Our spark circuit tester can be used on petrol engines and some motorcycles also. How to Use: Ground the alligator clip of the tester and put the other end in the spark plug wire. Now you can easily read the length of the spark through the little window. The longer the spark gap the stronger the coil.