How to Diagnose and Replace a Faulty Ignition Coil

by:Haiyan     2023-08-27

How to Diagnose and Replace a Faulty Ignition Coil


The ignition coil is an essential component of the ignition system in a vehicle. It is responsible for converting the low voltage from the battery into the high voltage needed to ignite the fuel and air mixture in the engine. A faulty ignition coil can lead to misfires, decreased performance, and even engine damage. In this article, we will guide you through diagnosing and replacing a faulty ignition coil, ensuring your vehicle runs smoothly and efficiently.

I. Understanding the Ignition Coil:

The ignition coil is a compact electrical transformer that consists of two coils of wire, the primary and secondary windings. The primary winding receives power directly from the battery, while the secondary winding generates the high voltage required for the spark plugs. When the ignition switch is turned on, the primary winding produces a magnetic field. This field collapses rapidly when the ignition switch is turned off, causing a high voltage induction in the secondary coil, which is then directed to the spark plugs.

II. Symptoms of a Faulty Ignition Coil:

1. Engine Misfires: One of the most common signs of a faulty ignition coil is engine misfires. Misfires lead to a rough running engine and a noticeable decrease in power. The misfires can occur at idle, during acceleration, or under heavy load.

2. Rough Idle: If your engine is idling roughly or shaking, it could be due to a faulty ignition coil. The coil may not be providing sufficient voltage to the spark plugs, resulting in uneven combustion.

3. Decreased Fuel Efficiency: A faulty ignition coil can negatively impact fuel efficiency. The lack of a strong, consistent spark can lead to incomplete combustion, wasting fuel and reducing mileage.

4. Difficulty Starting the Engine: If you experience difficulty starting your vehicle, especially when the engine is warm, it could be a sign of a failing ignition coil. The coil may not be able to provide the required voltage to start the engine.

5. Engine Stalling: A faulty ignition coil can cause intermittent engine stalling. The engine may suddenly shut off while driving, making it inconvenient and potentially dangerous.

III. Diagnosing a Faulty Ignition Coil:

Before replacing the ignition coil, it is important to confirm whether it is indeed faulty. Follow these steps to diagnose the issue:

1. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to check for any stored DTCs related to the ignition system. If there are any codes related to a faulty ignition coil, it is likely the culprit.

2. Visual Inspection: Inspect the ignition coil for any visible damage, such as cracks, corrosion, or a damaged wire. Physical damage can often lead to coil failures.

3. Resistance Testing: Use a digital multimeter to measure the resistance of the primary and secondary windings of the ignition coil. Consult your vehicle's service manual for the appropriate resistance values. If the resistance is outside the specified range, the coil is likely faulty.

4. Spark Testing: Remove one spark plug wire from the spark plug and insert a spark tester into the end of the wire. Start the engine and check for a strong, blue spark. Weak or no spark indicates a faulty ignition coil.

IV. Replacing a Faulty Ignition Coil:

Once you have determined that the ignition coil is faulty, follow these steps to replace it:

1. Gather the Required Tools and Parts: You will need a new ignition coil, a socket set or wrench, a screwdriver, dielectric grease, and a new set of spark plugs if necessary.

2. Disconnect the Battery: Ensure the ignition switch is in the off position and disconnect the negative terminal of the battery to prevent any electrical shocks.

3. Locate and Remove the Faulty Ignition Coil: Depending on your vehicle, the ignition coil may be located on the engine or inside the distributor cap. Disconnect the electrical connector and remove any mounting bolts or screws to release the coil.

4. Install the New Ignition Coil: Apply a thin layer of dielectric grease to the end of the ignition coil that connects to the spark plug wire. Position the coil in place and secure it with the mounting bolts or screws. Reconnect the electrical connector.

5. Reinstall Spark Plugs and Test: If you had to remove the spark plugs, replace them with new ones and torque them to the manufacturer's specifications. Reconnect the spark plug wires. Finally, reconnect the battery and start the engine to ensure the new ignition coil is functioning correctly.


Diagnosing and replacing a faulty ignition coil is an important aspect of vehicle maintenance. By recognizing the symptoms and following the steps outlined in this article, you can effectively diagnose and replace a faulty ignition coil, ensuring optimal engine performance and reliability. Remember to always consult your vehicle's service manual and exercise caution when working with the electrical components of your vehicle.

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