The spark plugs are the business end of the ignition system. The spark plugs deliver the spark needed to ignite the air/fuel mixture. No spark means no combustion, wasted energy, increased emissions, loss of performance, idle roughness, hesitation, hard starting and possibly even a no start if all of the plugs are affected.

 Optimal spark plug appearance

The insulator nose is of a grey-white/grey-yellow to light red-brown colour. The electrode erosion is low. The thermal value of the spark plug has been chosen correctly. No thermal overload. The mixture and ignition settings are correct, the engine is in good condition.

In modern systems, the distributor is omitted and ignition is instead electronically controlled. Much smaller coils are used with one coil for each spark plug or one coil serving two spark plugs (for example two coils in a four-cylinder engine, or three coils in a six-cylinder engine).

These coils may be remote-mounted or they may be placed on top of the spark plug (coil-on-plug or Direct Ignition). Where one coil serves two spark plugs (in two cylinders), it is through the "wasted spark" system. In this arrangement the coil generates two sparks per cycle to both the cylinders. The fuel in the cylinder that is nearing the end of its compression stroke is ignited, whereas the spark in its companion that is nearing the end of its exhaust stroke has no effect. The wasted spark system is more reliable than a single coil system with a distributor and cheaper than coil-on-plug.

Where the coils are automatically mounted they may all be contained in a single moulded block with multiple high-tension terminals. This is commonly called a coil-pack.

Spark plug wires (also called "ignition cables") carry high voltage current from the ignition coil(s) to the spark plugs. On older engines with distributors, the wires run from the distributor cap to the plugs, and are different lengths to reach the closest and furthest spark plugs.

The wire set also includes an extra wire that connects the center terminal on the distributor cap to the ignition coil (unless the application has the coil mounted in the distributor cap. (GM HEI )

On engines with distributorless ignition systems (Waste Spark ), plug wires are also used to connect the coils to their respective spark plugs. On some engines, there is one coil per spark plug, while on others with "waste spark" ignitions, two plugs share each coil.

On coil-on-plug (COP) (Ford Triton )ignition systems, there are no plug wires because the coils are mounted directly on top of the spark plugs. But in coil-near-plug (CNP)  systems, there are short wires that connect the coils and plugs. On some applications, these wires are part of each coil assembly and cannot be replaced separately.