Top Tips: Misfires and ignition coils

Before coil packs came along

Before the modern coil pack and ignition coil technology, most will remember or have heard of the 'distributor' within an engine.

This device was simply a rotating arm that passed over a number of metal contacts as it rotated. As it passed over each contact, the high-voltage from the ignition coil would be 'distributed' to the correct spark plug to generate the spark.

The move to coils and coil packs

Modern engines have done away with the distributor, instead using low-voltage electronic signals from the engine-management system to trigger the spark.

These signals are sent to an individual coil for each spark plug. These coils act as a transformer to generate the high-voltage required to operate the spark plug.

The coils convert the 12 volt signal from the car internal supply to the 25,000 to 30,000 volts needed by the plug to create the spark.

By using individual coils, the need to 'switch' high voltages is removed. This was often a source of trouble in earlier engines - especially in damp conditions, as I'm sure many will remember.

Spotting a fault

Coils and coil packs fail for a variety of reasons.

This often manifests itself as a poor idle, intermittent misfires at low engine speed or a total failure of spark in the affected cylinders.

Engine diagnostic tools are key to helping identify the source and location of the fault. They will often not only identify that the fault is present, but also which coils have failed.

Common fault codes pointing towards possible ignition coil faults are, for example:

P0351 - Ignition coil A, primary/secondary - circuit malfunction

P0303 - Cylinder 3 - misfire detected

P0300 - Random/multiple cylinder(s) - misfire detected

Fixing the fault

Replacement coils and coil packs are readily available and are generally quick and easy to change.

Coil setups can vary between cars, but there are 3 main types:

pencil: where separate coils are located very near to or directly on top of each spark plug (known as direct ignition or coil-on-plug)

block: where the coils are mounted together in a single 'coil pack'. This block is mounted away from the spark plugs and is attached to them with HT leads

cassette: again where the coils are mounted together as a pack, but the cassette generally fits directly onto the spark plugs

Check our online application list for your car - we currently stock replacement coils and packs for over 600 car models.