It is time to retire the spark plug tester!
A spark tester shows if there is secondary spark, but, “the scope can give you the value of that spark as well as the duration, the input voltage and the oscillations associated with the signal,” says Scott "Gonzo" Weaver in this article from the March 2015 issue of UnderHood Service.
The key is to understand the secondary waveform shown below. Read how 'Gonzo' does it in this short article.
