Ignition coil: problems, when to replace, repair costs
Ignition coil in a Ford Ecoboost engine cutaway
An ignition coil is a part of the vehicle's ignition system. An ignition coil provides a high voltage in order to create a spark at the spark plug. The spark ignites the air/fuel mixture in the engine cylinders.
Most modern cars have one ignition coil per cylinder. Often, an ignition coil is installed right over the spark plug, like in this Ford engine in the photo. This setup is called coil-on-plug. In some cars, ignition coils for all cylinders are combined into one coil pack. In older cars with a distributor, a single coil is used for all cylinders. See this example of a single ignition coil in a Volkswagen air-cooled engine.
Ignition coil problems
Ignition coils are common to fail in many cars. Symptoms of a bad ignition coil include engine shaking, sputtering and lack of power (misfiring). The Check Engine light may flash repeatedly or stay on solid on the dash. In cars with a single ignition coil, a bad coil will cause the engine not to start.
Problems with ignition coils are more common in rainy weather or after water has gotten into the engine bay. Ignition coils are more likely to fail if spark plugs haven't been replaced for a long time. This is because an old spark plug has a larger gap and higher resistance, which adds strain on the ignition coil. In some cars, driving with a failed ignition coil can damage a catalytic converter or even the engine computer (PCM).
Ignition components safety precautions
An ignition coil produces extremely high voltage that is dangerous for your health. Whenever working with ignition components, follow safety precaution found in the service manual for your car. The first step when inspecting or replacing an ignition coil is to disconnect the negative battery cable.
How a bad ignition coil is diagnosed
First, your mechanic will scan the engine computer for trouble codes. The trouble code may point out which cylinder misfired or even which coil has failed. For example, the code P0301 means that cylinder one misfires. The code P0351 reads Ignition Coil "A" Circuit Primary/Secondary Malfunction (cylinder 1). After that, an ignition coil must be inspected following the service manual procedure.
Usually, an ignition coil inspection includes measuring the resistance between specific ignition coil terminals. If the resistance is outside of specifications, the ignition coil must be replaced. Sometimes to identify a failing ignition coil, your mechanic may try swapping a suspected coil with a known good one, to see if the cylinder stops misfiring. Ignition coils are also recommended to be replaced if they show cracks, traces of arcing or other damage that may cause it to short out.
Ignition coil replacement
On-plug ignition coil
A faulty ignition coil cannot be repaired; it must be replaced. In most cars with a 4-cylinder or inline-6 engine, replacing an ignition coil is a simple and not very expensive job ($180-$380 for one coil).
In some V6 engines, to gain access to rear ignition coils, an intake manifold must be removed (e.g. Ford Edge, Ford Escape V6, Nissan Murano, Toyota Camry V6). This involves more labor and costs more ($280-$500 for one rear coil). In cases like this, to prevent future problems, it make sense to replace all three rear ignition coils.
Whenever one of the ignition coils goes bad, it's also recommended to replace all the spark plugs if they haven't been replaced for a long time. New spark plugs will help ignition coils last longer.
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