An ignition coil is inside the ignition assembly and transfers the 12-volt electrical signal from the battery into the voltage needed to make the spark plugs spark. Because the ignition coil is responsible for making the plugs spark, it is sometimes called a spark coil. When an ignition coil goes bad, it can manifest itself with one or more signs and symptoms.
If the ignition coil goes bad, the battery’s 12-volt current will not be converted for the spark plugs, and no spark will occur. Without spark, the engine will not start.
If the coil is going bad but is still marginally functional, it will provide electrical current for some of the plugs, but not all. This problem may be mistaken as a bad spark plug. If the weak coil cannot maintain a spark for all the plugs, the vehicle will run poorly and may buck, feel sluggish, fail to accelerate properly, run loudly and shake.
If the coil is bad but still functional, the car may stall one day while running and not start again. This happens when it goes completely bad.