Ignition Coil

Most modern petrol engines have one coil per cylinder sitting directly on the spark plug (coil-on-plug). The coil can produce tens of thousands of volts for an instant. Coils fail from heat, vibration and age, and a worn spark plug with a large gap makes them work harder. A failing coil usually causes a misfire on one cylinder.

Most modern petrol engines have a separate coil on each spark plug, called coil-on-plug. Older engines used a single coil and a distributor, or a coil pack for two cylinders.

Current flows through the primary winding and builds a magnetic field. When the control unit cuts the current, the field collapses and induces high voltage in the secondary winding, firing the plug. The control unit switches current through the primary winding; when it switches off, the collapsing magnetic field creates a high voltage in the secondary winding.

A bad coil causes a misfire code for its cylinder, rough running and a flashing engine light under load. Swapping the coil with another cylinder is a common test: if the misfire moves, the coil is faulty. A failing coil causes misfires, often under load or in damp weather. Swapping coils between cylinders and seeing whether the misfire moves is a common way to confirm it.