Many modern cars have spark plug replacement intervals of close to (or even greater than) 100,000 miles. However, the majority still require more frequent replacement, so if the plugs in your car haven't been changed in a while and engine performance is down, a new set should be a part of your next service package.

To perform a quick check of the plug leads, turn off the light in the garage and run the engine with the hood partly closed to darken the area. Arcing at either end of the wires is an indication that they need replacement. Another way to test plug wires is to simply spray them with water, which will promote arcing through wires with weak insulation. If even just one wire shows weakness, the safest practice is to replace them all, duplicating the original routing through the on-engine plug wire guides. Today, more and more engines are going to coil-on-plug ignition, which eliminates conventional plug leads altogether.

Here, the coil actually is part of a module that sits atop a spark plug boot. Over time, oil or water seepage combined with stray voltage can cause a failure in a coil-on-plug set-up. Expense usually dictates replacement of a single unit and that is usually sufficient.