Your spark plugs are like a cipher.
And to read a cipher code, you usually need a cipher key. That’s what this is—a cipher key that is going to help you “read” your spark plugs and understand what they’re telling you about how your engine is running.
Is your engine running too rich? Too lean? Could there be a problem with ignition timing, or your oil system?
The answers are often in your spark plugs, and by using this chart to diagnose your engine’s performance, you’ll be able to make whatever adjustments might be necessary to achieve peak performance.