Ignition coil
Definition
The ignition coil is a step-up transformer that turns battery voltage into the 20-40 kV needed to jump the spark plug gap; the ECU or an ignition module switches the primary circuit at the right moment. Coil-on-plug designs put one coil on each plug, so a failed coil gives a steady misfire on a single cylinder, while a wasted-spark pack serves two cylinders. Coils fail from heat, oil leaking into the plug well from a valve cover gasket, cracked insulation that arcs to the head (visible in the dark), and worn plugs that force higher secondary voltage. Diagnose by swapping the suspect coil to another cylinder and seeing whether the misfire code follows it. Codes: P0351-P0362 (coil primary/secondary circuit by cylinder), P0301-P0312 (cylinder-specific misfire), P0300 random misfire.