Car Tips: How to Know if It’s Your Spark Plugs or Ignition Coil That’s Bad

Over time, all engines experience a gradual reduction in power output, but consistent maintenance plays a crucial role in mitigating this decline. Nevertheless, irrespective of an engine’s size, whether large or small, it remains unable to generate any horsepower without the essential components of fuel, air, and a spark.

Issues like clogged fuel injectors and air filters can present challenges for vehicles, including trucks, cars, and motorcycles, and so can malfunctioning spark plugs. However, these issues are not the sole culprits when it comes to a lack of sparks. Occasionally, what appears to be a spark plug problem may actually stem from an issue with the ignition coil.

Exploring How Ignition Coils Work in Conjunction with Spark Plugs

While spark plugs and ignition coils serve distinct purposes, they are integral components of a vehicle’s ignition system, specifically in gasoline-powered vehicles. Diesel engines, in contrast, do not utilize spark plugs, as their ignition process relies on compression rather than a spark.

The evolution of ignition systems has occurred over several decades, yet their fundamental operation remains largely consistent. Ignition systems comprise two primary components: the primary ignition and the secondary ignition. The primary ignition engages when you initiate the engine by pressing the start button or turning the key. It completes an electrical circuit, facilitating the flow of power from the battery to the ignition coil and back, resulting in the formation of a magnetic field within the ignition coil.

Subsequently, the secondary ignition system comes into play. The specifics of this mechanism can vary depending on whether the vehicle employs a conventional system with a distributor, a more contemporary distributor-less setup, or the latest coil-on-plug system. Nevertheless, the outcome remains consistent—the ignition system momentarily interrupts the magnetic field within the ignition coil.

One expert explains that an ignition coil essentially functions as a wire-based transformer. Simplified versions consist of two coils of wire wrapped around an iron core. When the magnetic field is disrupted, it induces an electrical current between the inner and outer coils, amplifying the voltage from the battery from 12 volts to a substantial 50,000 volts or more.

This high voltage is indispensable because it empowers the spark plugs to perform their crucial function. Without such elevated voltage levels, the electrical current would be unable to bridge the gap between the electrodes of the spark plug.

Identifying Signs of Faulty Ignition Coils and Spark Plugs

Both spark plugs and ignition coils are vital to the proper functioning of an engine. Failure of either component can render your car, truck, or motorcycle inoperable. However, pinpointing which one is causing the issue can be challenging. Several symptoms can be indicative of problems with either, as explained by Autoblog.

For instance, signs such as engine misfiring, power loss, or rough idling may suggest a faulty spark plug, according to Autoblog. However, these same symptoms could also point to a malfunctioning ignition coil. The illumination of the check engine light or difficulties in starting the engine can also be indicators of issues with either component.

In earlier ignition systems, a failure to start the vehicle often signaled a problem with the starter or ignition coil. This was because early ignition systems relied on a single ignition coil that supplied all the spark plugs via a distributor, as reported by Haynes. However, more modern distributorless systems replaced the distributor with a coil pack and multiple coils. The current standard, known as coil-on-plug, assigns each spark plug its individual top-mounted ignition coil.

If an ignition coil is faulty, the spark plug it is connected to will also experience impaired functionality.

Can You Address This Issue at Home?

There are methods to diagnose a faulty ignition coil, regardless of the ignition system in your vehicle, according to experts. One straightforward approach involves using specialized spark testers or test lights, designed for both distributor-based systems and modern coil setups. These tools attach directly to the wiring and indicate when an electrical charge flows through them.

Another common diagnostic method involves connecting a multimeter and assessing the resistances of the inner and outer coils. If the resistance falls outside the range specified by the manufacturer, it indicates the need for ignition coil replacement. However, it is possible for defective coils to pass this test. It’s important to note that damaged spark plugs and plug wires can also negatively affect ignition coils, not just the other way around.

A simple way to test whether the issue lies with the coil or the spark plug is by swapping these components. Since spark plugs often need replacement, connecting the suspected ignition coil to a new plug with new wiring can help identify the source of the problem. If the engine still fails to fire, it indicates an issue with the coil. Alternatively, you can disconnect the wires from each cylinder one at a time. If the engine’s idle RPM remains unchanged, it suggests that the coil for that cylinder is deteriorating.


Fortunately, for vehicles equipped with a coil-on-plug system, replacing the coils is no more complex than changing the plugs themselves. Older distributorless ignition designs may require the removal of an engine cover, as reported by Haynes, typically secured with a few screws. However, it’s crucial to keep track of which spark plug wire connects to which coil in the case of a coil pack, as incorrect connections can disrupt the engine’s ignition timing and lead to further complications. If you’re uncertain about handling this task, it’s advisable to seek the assistance of a qualified mechanic.

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