What does a secondary ignition waveform show?
Short answer: A secondary ignition waveform shows firing voltage, spark line voltage and burn time, which reveal problems such as worn plugs, wide gaps, lean cylinders, or coil and wire faults.
The secondary waveform starts with a firing line, the voltage needed to jump the plug gap. Then a spark line shows the voltage that sustains the spark and how long it burns. After the spark ends, coil oscillations appear.
A high firing line suggests high resistance, such as a worn plug, wide gap or lean mixture. A low firing line can point to a fouled plug or low compression. A short burn time suggests energy is being used up too quickly, such as by high resistance in the secondary path. Comparing cylinders is often more useful than absolute numbers.
On coil-on-plug engines, a capacitive probe or secondary adapter picks up the signal through the coil. Never pull a coil off a running engine to look for a spark, since secondary voltage can cause a painful shock and damage the coil. Use the right probe and keep leads clear of hot exhaust parts.
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General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.