Ignition coils provide the spark that fires each spark plug in an engine. The ignition coil takes the 12 volts from the battery and converts it to as much as 45,000 volts, supplying enough voltage to create a spark. Ignition coils are either linked to the plugs using spark plug wires or cables, or each coil is directly connected on top of each spark plug, which is known as a coil-on-plug (COP) system. COP ignition systems are found on most of today's vehicles with one ignition coil per cylinder. Some older vehicles use a coil pack or single ignition coil to fire all spark plugs.
Before the advent of computer control for ignition, most vehicles still used a mechanical distributor and plug wires to manage spark for each cylinder. The voltage for the spark came from an ignition coil mounted off to one side from the distributor. Unlike a transformer that raises or lowers voltage, an ignition coil uses oscillation of a magnetic field to produce voltage and is comprised of layers of copper windings separated by layers of oiled paper for insulation.
Problems with the ignition coil can arise from worn, eroded spark plugs that are putting too much of a demand for voltage on the whole system. If the coil is starting to fail, it can manifest as backfiring, poor performance and poor power from a weak spark. Shop Advance for ignition coils, modern coil-on-plug coils and even distributorless coils (a transitional system you'll find on some cars from the 90s and 00s).
Learn more about the different types of ignition systems and how they work.