The 9 Best Spark Testers

Magazine reviewers and forum regulars have pointed to the Lisle 20610 for years as a plain way to check for spark. It slips between the plug and its wire, and a bulb in the clear body flashes with each spark while your fingers stay off the live lead. Straight and angled boots fit most plug wires, and Lisle sells a replacement bulb. The flash shows spark is there, not how strong it is, and coil-on-plug engines need an extra lead.

Kept in a lawn-mower toolbox or a glove box, the Ram-Pro 2331 is a cheap spare for the few times a year a dead engine raises the question of spark. It goes between the plug and its wire, and a bulb behind the clear housing flickers whenever spark gets through. The bulb can be swapped through that clear shell. It's a seller's brand with no maker spec page, many look-alikes copy it, and a flash says nothing about spark strength.

Many newer cars have a coil sitting on each plug, with no plug wire to tap, and the Lisle 20580 kit is made for them. Besides the bulb tester, it brings a lead for engines with plug wires and a second lead that fits between the coil and the plug. One kit covers many newer cars, older engines and small motors, with a limited lifetime warranty. The bulb still shows only whether spark is present.

To see how far a coil can push a spark, reach for the Lisle 20700. Clip its ground wire to clean metal, connect the tip to the coil or plug wire and widen the gap with a knob while the engine cranks. The tip unscrews for small engines and some motorcycles. An open gap can shock you, too wide a gap can strain a coil, and fuel vapor must stay away. The threaded tip can also sit loose.

Small-engine owners who want proof, not a flicker, can turn to the Oregon 42-087, modeled on a Briggs & Stratton design. The spark has to jump a fixed gap wider than a normal plug's, so a spark that crosses it means the ignition works. You put the plug wire on one end, clip the lead to bare metal and pull the starter. The exposed metal can shock you, and the instructions are thin.

Plugs buried deep in the head, out of reach of a stiff in-line tester, are the job for the Tool Aid 23970. The kit pairs a flashing bulb tester with a long lead, an alligator-clip ground and boot adapters, all in safety orange so the pieces are hard to lose under the hood. It works on older ignitions and coil-on-plug setups alike. Some pieces can break early, and a bulb can't judge spark strength.

Some technicians prefer a fixed-gap tester like the OTC 6589 for electronic ignitions. Shaped like an oversize spark plug with a built-in ground clip, it takes the plug wire, clips to metal and shows the spark while you crank. Because the spark must jump a wider gap than a real plug's, a weak coil that would still light a bulb can fail here. It isn't cheap, and coil-on-plug engines need an extension.

You can watch the spark jump inside the clear tube of the Thexton THE404, a small adjustable-gap tester. An alligator clip holds it to ground, and widening or narrowing the gap gives a rough idea of spark strength, a check a bulb can't make. It comes with a one-year warranty. The spark can be hard to see in bright sun, and as with any open gap, a shock is possible.

Before buying a full set of new plugs, you can use the ANCEL SP02 to compare a used plug against a fresh one on the bench. Two plugs sit in its sockets under a clear cover, powered from a 12-volt source, and a knob mimics different engine speeds while you watch the sparks side by side. A plug that sparks in open air can still misfire under compression, and the box can't check coils or wires.