Review on ABN Inline Spark Plug Tester Tools & Equipment by Tio Kotaki

You get what you pay for. One of the testers didn't work right away, which doesn't inspire much confidence since, as simple as these devices are, you expect them to work. A working tester allowed me to check for sparks, which saved me a bit of time pulling out the spark plugs to check them, although I ended up pulling them out anyway. I assume it's a spark test gap, if you don't have time to unplug that's the price. Not a bad tool when it works.