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Modern automotive ignition systems use coil packs for every cylinder, shortening wire runs, increasing spark energy, and improving timing. However, coil-on-plug ignition systems, with their short or non-existent wires, have rendered some proven spark plug tests obsolete. It’s easy to test for spark, but Thexton’s 458 plug tester can also tell you how strong a spark you’re getting.

The tester has an adjustable gap between the anode and cathode, and since air’s electrical resistance is nearly constant, adjusting the gap can give a mechanic a good qualitative picture of a voltage’s magnitude. The 458 is adjustable from 0V to 40,000V, which, while lower than the spark energies of many common COP systems (for instance, GM’s LS engines have 55,000V packs), is more than enough to ignite a good fuel/air mixture.

Though spark technology has evolved, the tools haven’t gotten much more expensive. The Thexton is available from ToolSource.com for just under $19 before shipping. As an added bonus, it looks like it’ll work with distributor-based and wasted-spark ignition systems as well.

Thexton 458 [Tool Source]

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4 Responses to Modern Spark Plug Tester

  1. JB says:

    This is kind of scary. A COP or CNP system can potentially put out enough voltage and amperage to kill you. To test these systems properly you need an oscilloscope to test the primary side of the ignition coil. The pattern will look the same and show the same issues but it will be hundreds of volts as apposed to hundreds of thousands of volts.

  2. Toolhearty says:

    JB Says:

    This is kind of scary. A COP or CNP system can potentially put out enough voltage and amperage to kill you…

    Haven’t checked, but I’m pretty sure that just because the coil is now on or near the plug doesn’t mean the coils are producing any (or significantly) more power than they have traditionally. There’s always been enough juice to kill you under the right circumstances.

  3. Lex Dodson says:


    Plenty of my former co-workers have taken hits from COP systems (sometimes in very private places), and they’re all still alive. When it comes to electrical shocks, it’s not the voltage that kills, it’s the amperage. I wouldn’t be surprised if an industrial 480V junction box is more dangerous than a lightning bolt.

  4. Always take safety precautions before attempting certain activities. Voltage and amperage are both dangerous. One false move can’t be undo to restore the damage.

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