When a spark plug isn't properly firing in a specific cylinder, it's known as a misfire. This may cause the engine to run rough, shake faster or even stop running altogether if it's not fixed soon enough. Misfires are common and can be caused by a number of different things, including fuel injector problems. If your car is still experiencing a misfire after you change the spark plugs and ignition coil, there are a few things to check that may help.
Ignition coils are responsible for the electrical circuits that produce the high-voltage electricity needed to ignite the air/fuel mixture in your engine. This voltage travels through the coil's primary winding, causing a magnetic field that induces oscillations in the secondary winding. The result is the generation of a spark that then causes your fuel injector to deliver the proper amount of fuel to a specific cylinder.
While ignition coils typically have a long lifespan, they can start to malfunction over time, which is when you'll start to notice that your engine is running poorly. When that happens, your engine management system will likely detect the problem and turn on your check engine light.
To test an ignition coil, you can use a multimeter or ohmmeter to measure the resistance of each of the terminals on the coil. You'll want to look for a coil with a lower reading than the rest to identify which one is faulty. Once you've found the faulty coil, you can replace it with a new one.