Signs of a Failing or Bad Ignition Coil

Ignition Coil

The ignition coil is an integral part of the ignition system that converts the electrical voltage from the battery to the amount needed for the spark plugs to ignite the fuel/air mixture in the engine cylinder’s chamber. Without them, the spark plug would not ignite the mixture and combustion would not happen.

Over time, ignition coils have changed and there are different types of ignition coils and ignition systems, so symptoms of a bad ignition coil are not universal. Many cars today use electronic ignition with direct ignition, also known as coil-on-plug ignition systems, so we’ll cover the symptoms of a bad ignition coil that uses the coil-on-plug ignition system for each individual spark plug and its variation, the wasted spark ignition system, which has an ignition coil for multiple spark plugs.

Check out this article for more on the different types of ignition systems.

Symptoms of Bad Coil-On-Plug Ignition Coils

The most common kind of ignition coil found on vehicles today, coil-on-plug ignition coils that sit directly over the spark plug, and ignition coils in wasted spark systems that manage multiple spark plugs, display a few different symptoms when they’re beginning to wear or are defective. Common symptoms include:

1. Check Engine Light

On many modern vehicles, when an ignition coil goes bad, a check engine light will come on. This may be accompanied by other symptoms like a rough running engine. The light may also blink, which indicates that further usage of the engine can damage the catalytic converter. A positive of this symptom is that OBD-II scanners can identify which cylinder is misfiring, which can help with diagnosing engine misfires and specifically bad ignition coils.

The code can read as P0300, which means a random misfire has been detected but it may not be from defective ignition coils. If there is a number at the end of the code (ex. P0301), this indicates a misfire is happening at a specific cylinder (in this case cylinder 1) and the ignition coil can be switched with another to see if a bad ignition coil is the cause.

2. Decrease in Engine Performance

Before the ignition coil is completely defective, other signs like a drop in gas mileage or slow accelerating and less engine power may happen. These may indicate the ignition coil or other ignition parts aren’t performing. For example, if the ignition coil fails to consistently deliver enough voltage to the spark plug, the electronic control unit (ECU) may send more fuel to the engine, resulting in a drop of miles per gallon (mpg).

3. Rough Running Engine

A very noticeable symptom of a bad ignition coil is a rough running engine. One day you might be driving with the engine running fine when it starts to rumble intermittently —as if it’s jerking or struggling to consistently run as smooth as it normally does. This is usually the symptom of a misfire, which could mean that you have a bad ignition coil.

4. One or More Cylinders Misfire

Because the ignition coil provides voltage for two spark plugs in a wasted spark system, two cylinders can misfire if the ignition coil is defective.

Why Do Ignition Coils Fail?

1. Age

Over time, ignition coils can wear out. Since they are subjected to high voltage, their internal components have the ability to wear. Ignition coils can also be damaged from internal leaks like engine oil or can be overworked by worn spark plugs.

2. Heat

A consequence of the high voltage ignition coils experience is heat. This can happen if the ignition coil is forced to work harder, like providing more voltage for a bad or misgapped spark plug. An overheated ignition coil can eventually cause cracking, underperformance and failure.

3. Corroded Electrical Connection

Sometimes ignition coils fail because the electrical connectors can break, melt, and corrode over time. This can be confirmed when removing the ignition coil by checking for discolored, melted, or broken electrical connectors.

4. Other Worn Ignition System Parts

Other worn ignition system parts like worn spark plugs can cause the ignition coil to work harder and supply more voltage than it’s been designed for, wearing out the part’s internal components.

How to Test Bad Ignition Coils

There are some DIY methods for diagnosing ignition coils in wasted spark and individual coil-on-plug ignition systems. Wasted spark ignition coils specifically can be tested with a bottle of spray water, which consists of placing the vehicle in a dark area, lightly spraying the coils with water, and turning the vehicle on to see if there are sparks. Both wasted spark and individual coil-on-plug ignition coils can be tested by rearranging the ignition coils to see if they are the cause of a misfire in a specific cylinder. To learn more about diagnosing ignition coils yourself, check out this article.

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