Ignition Coil vs. Spark Plug: What’s the Difference?

The main difference between an ignition coil vs. spark plug is in function—the coil creates the spark that the plug uses to ignite your engine’s fuel.
Written by Andrew Biro
Reviewed by Melanie Reiff
Though they are both parts of your vehicle’s ignition system, the ignition coil and spark plug do not share the same function. The coil transforms low voltage from the battery into high voltage, creating an electric spark in the plug, which is then used to ignite fuel.
Chances are you’ve heard of a spark plug, and maybe even an ignition coil, too—but do you actually know what they are or what makes them different? Knowing how an ignition coil vs. spark plug works and the purposes they serve will help you to identify which one may be causing problems based on the symptoms you observe.
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Ignition coil vs. spark plug: what’s the difference?

While both are an integral part of your vehicle’s ignition system, the ignition coil and spark plug are not the same part, nor do they serve the same purpose. One supplies power while the other creates the actual spark needed for combustion. 

Ignition coil

ignition coil
is an induction coil located within a vehicle’s ignition system composed of an inner and outer coil. It helps create the spark utilized by the engine’s spark plugs.
When you turn the key to start your vehicle, the primary ignition is triggered and completes the circuit between the battery and ignition coil, sending power from one to the other and then back again. This creates a magnetic field in the coil.
After this, the secondary ignition is activated. It briefly interrupts the ignition coil’s magnetic field and creates an electrical current between the inner and outer coils. This transforms the battery’s 12 volts into 50,000+ volts.

Spark plug

spark plug
is the part that actually supplies the spark to ignite the fuel mixture in your engine, kicking off the combustion process that supplies your vehicle with the necessary power to move.
The spark plug uses the 50,000+ volts created by the ignition coil—after the plug receives this power, an electrical current jumps across the plug’s electrode gap and creates a spark that helps light up the fuel in your engine. 
MORE: Why isn’t my car starting? A quick troubleshooting guide

Symptoms of a bad ignition coil vs. spark plug

Now that you’ve learned the differences between the ignition coil and spark plug, you might be inclined to believe that they also differ in symptoms once they start to go bad—but that isn’t always the case. 
Distinguishing between a faulty coil or spark plug can be incredibly difficult, especially since a bad ignition coil can lead to spark plug failure as well.
In this case, it’s probably best to take your vehicle to a mechanic and have them determine which component is at fault. Here are the symptoms to watch out for: 
  • Difficulty starting
  • Engine misfires
  • Loss of power
  • Worsening fuel economy
  • Car hesitates or vibrates while idling
Of course, there are several things you can do yourself to narrow down which part is at fault, with the easiest being to swap out parts. Regardless of which part has gone bad, you’ll almost always need to replace the spark plug—so if you swap out your old plugs and find that the problem persists, you’re definitely dealing with a bad ignition coil.
You can also test the ignition coil directly by hooking it up to a multimeter and testing the resistance of the inner and outer coils. If it comes up with a value outside the range specified by the manufacturer, the coil will need to be replaced.

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No, your ignition coil and spark plug are not the same thing, nor do they serve the same function—though they both play a crucial role in the ignition system. 
The ignition coil transforms low voltage from your car battery into high voltage. The spark plug takes the spark created by the coil to ignite the fuel in your engine.
It depends. As both parts show the same or similar symptoms when they become faulty, narrowing down which is at fault can be tricky. 
Chances are that your spark plugs will have to be replaced regardless—meaning you can simply buy new ones, switch them out, and see if the problem persists.
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