Dual-plug ignition coils are part of what in powerspors is the almost ubiquitous "waste spark" ignition system, a technology you car guys would know better as the DIS (distributorless ignition system) ignition in the emerging four-cylinder autos of the 1980s. Compact, adequate, and a reasonable performer, the waste spark system fires the one coil's two plugs in series, one igniting a cylinder's compressed mixture, the other firing needlessly in the other cylinder's exhaust. This is so that 180 degrees later they can swap their roles and thus complete ignition on two cylinders using just one coil. A neat system.

The theory would convince you that if one of the coil's spark plugs failed to fire, neither would fire, since the two are in series. But this is not true, as any career mechanic knows.

There are numerous ways to explain how one plug might fire and the other not. For example, a fouled plug. A spark plug that is fouled will, though not able to spark, still conduct current through the conductive carbon, offering no break in the electrical path. In fact, the firing plug will actually have more voltage available to it. Another way one plug might fire and not the other is when one plug for whatever reason won't conduct, and thus an electrical strain is built up inside the coil. Often enough this strain will result in an arc between the primary and secondary windings inside the coil, thus completing the circuit and allowing the good plug to fire. That this is a real possibility with any ignition coil is why it is bad practice to leave ignition coils "hot" yet the plugs removed from their wires. It can damage the coils.