how to tell if ignition coil is bad

by:Haiyan     2023-07-02

Ignition Coils: How to Tell if They Are Bad


Ignition coils are an integral part of the ignition system in your vehicle. They play a crucial role in transforming the low voltage from the battery into the high voltage needed to ignite the fuel-air mixture in the engine cylinders. Over time, ignition coils can deteriorate or fail due to various factors, affecting the overall performance of your vehicle. In this article, we will explore how to tell if your ignition coil is bad and the potential signs you should look out for.

Understanding Ignition Coil Function:

Before diving into the signs of a bad ignition coil, it is essential to understand its basic function. The ignition coil is responsible for generating the high-voltage spark that ignites the air-fuel mixture in the engine. When you start your vehicle, the battery sends a low-voltage current to the ignition coil. The coil then amplifies the voltage and transmits it to the spark plugs, creating a spark that ignites the fuel and powers the engine.

Signs of a Bad Ignition Coil:

1. Engine Misfires:

One of the primary indicators of a faulty ignition coil is when your engine begins misfiring. Misfires occur when the spark plugs fail to ignite the fuel-air mixture at the correct time. If you notice your engine vibrating, hesitating, or losing power, it may be due to a malfunctioning coil.

2. Rough Idle:

A bad ignition coil can result in a rough idle. If your engine sounds rough and occasionally stalls when idling, it may be due to an inconsistent spark created by a faulty coil. A healthy ignition coil ensures a smooth and steady idle.

3. Difficulty Starting:

When an ignition coil is failing, starting your vehicle may become a challenge. You might experience increased cranking time or even a complete failure to start. Since the ignition coil is responsible for providing the spark needed to start the engine, a malfunctioning coil can hinder the starting process.

4. Decreased Fuel Efficiency:

A deteriorating ignition coil can also have an impact on your vehicle's fuel economy. If you notice a sudden decrease in gas mileage, it could be attributed to a bad coil. Fuel efficiency can be compromised when the spark plugs are not firing correctly due to a weak or intermittent spark produced by the coil.

5. Check Engine Light:

In many cases, a faulty ignition coil triggers the check engine light. Modern vehicles are equipped with onboard diagnostic systems that detect irregularities in various components, including the ignition system. When the system detects an issue with the ignition coil, it illuminates the check engine light on your dashboard.

Testing the Ignition Coil:

If you suspect a bad ignition coil based on the signs above, it is crucial to have it tested before replacing it. Here are a few common methods used to test the ignition coil:

1. Resistance Testing:

Using a multimeter, you can test the resistance of the ignition coil. This method determines if the coil's windings are within the manufacturer's specified range. Resistance values outside of the recommended range indicate a faulty coil.

2. Spark Testing:

Performing a spark test can help identify if the ignition coil is generating enough voltage. By removing one spark plug wire at a time and grounding it while cranking the engine, you can check if a spark is present. A weak or non-existent spark indicates a malfunctioning coil.

3. Swapping Coils:

In some cases, you can swap the suspected faulty ignition coil with another functional one. If the problem follows the coil to the new location, it confirms the coil is defective. However, exercise caution when performing this test to prevent damage to other components.


The ignition coil is a crucial component of your vehicle's ignition system. Recognizing the signs of a bad ignition coil can help you diagnose and resolve issues promptly. If you encounter any of the symptoms discussed in this article, it is advisable to seek professional assistance or conduct tests to determine whether an ignition coil replacement is necessary. Regular maintenance and addressing potential ignition coil issues in a timely manner can help ensure optimal engine performance, fuel efficiency, and overall reliability.

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