Due to my recent problem with weakly firing plugs, I'm looking at inline spark testers. I've seen three basic types:

The simple flashing one which goes in between the wire and plug with the plug in place:

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A slightly more complicated type with the addition of a ground connection:

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And a type that has an adjustable gap and is grounded:

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The middle one makes the following claim:

pinpoints if there is a spark plug, plug wire, mechanical, fuel injection or electrical problem. Shows a duplicate view of the ignition spark so technician can visually check condition of ignition system at each cylinder.

What types of problems can be detected with each type, and how do they function?

  • And then there are the "non-contact" type of spark testers...
    – JPhi1618
    Commented Dec 30, 2015 at 13:33
  • If available, a simple timing light can also be used to test spark. Maybe you have one laying around? :)
    – race fever
    Commented Dec 31, 2015 at 16:58
  • @racefever Actually, I do. Didn't think about that. Commented Dec 31, 2015 at 20:36
  • @RobertS.Barnes Then you are in luck. If its a typical timing light you will be able to tell weak spark from the others by how bright the timing light shines. :)
    – race fever
    Commented Dec 31, 2015 at 20:39

2 Answers 2


First, think of the possible failure modes:

  1. Spark plug is clogged with carbon or has an isolation error - it conducts current, but doesn't generate sparks, because the current flows through the carbon from electrode to electrode.
  2. Break inside the plug, e.g. broken / worn electrodes - plug doesn't conduct (and of course doesn't generate sparks)
  3. Cable has a break / short
  4. Coil doesn't generate HV pulses
  5. Coil generates weak HV pulses

The first device just tests if current is flowing out of the cable into the spark plug. So if you can see it flashing, coil and cable are fine, but it's still possible there's failure #1.

Number two is basically the same, but you can also connect it directly to ground. If it flashes when connected to ground, but not when connected to the plug, you have #2. With all that additional equipment, it may also be connected directly to the coil, without the cable to test for #3 and #4.

The third is used without plug and allows to measure the strength of the HV pulse. It should be strong enough to generate a spark up to a certain gap width. For example, 30kV could form a spark over a 1cm gap in dry air. So this device allows to test #5.

Note that none of these tests #1, but a plug can be easily replaced for testing. If the resistance between terminal an thread/frame measured by a multimeter and it's not infinite, then the spark plug is probably clogged with carbon.

#5 is a bit special. A ignition coil has thousands of turns to generate the high voltage. It is possible that the insulation between some of the turns fails, and so a short circuit is formed between those turns. As result, the voltage of the pulse is lower than normal. This "weaker" pulse may still be able to ignite the mixture in your cylinder most of the times, but under some conditions, it will not, or not so reliably. Typically, the motor will stall or run roughly at low RPMs / when idling, and starting the motor is not easy. This means, while the plug still generates sparks and the first two devices will flash, the spark will be too weak to ignite the fuel. And that's a bit difficult to measure.

The third device allows to measure the pulse strength by testing what's the maximum width of a gap which will still generate sparks. However, if this pipe is filled with ambient air, the width heavily depends on humidity. So, using this is more an estimation than a real measurement. Yet, you may compare all the coils of the vehicle to test if one is weak.

  • So the first device tests 2,3,4 and maybe 5 and is the most general. Why do you say you "may" still have failure #5? What are the ways to detect #1? How could it conduct current but not generate a spark? The current flowing into the plug has to go somewhere. Why do you say the third device "may" allow testing for #5? Commented Dec 30, 2015 at 11:05
  • @RobertS.Barnes: I've edited this. The #5 problem is a bit difficult, because the first two devices will still be flashing, indicating everything is fine. And about #1: The best ways is to change the plug.
    – sweber
    Commented Dec 30, 2015 at 13:17
  • So what you're talking about in #1 is regular carbon fouling like described here? ngk.com.au/spark-plugs/technical-information/… Commented Dec 30, 2015 at 14:07
  • The second one claims to also test fuel injection somehow - any idea how? Commented Dec 30, 2015 at 14:18
  • About fouling: yes, exactly. I didn't know the correct term. About fuel: the device can't test that. But when you have a problem and ignition passed the test with this device, the problem is something else, like injection.
    – sweber
    Commented Dec 30, 2015 at 14:59

You can waste hours of your time with Inline testers they do not accurately check the spark plug. The plug acts as an earth, as long as it has any weak spark or carbon the light will come on. A new cde pty ltd spark tester kit ( see you tube demo) is the only accurate way to check your spark plug. It just takes 2 minute.


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