Common Symptoms of a Faulty Car Ignition Coil and How to Diagnose Them

by:Haiyan     2023-09-07

Common Symptoms of a Faulty Car Ignition Coil and How to Diagnose Them


Your car's ignition coil plays a crucial role in providing the spark necessary to start the engine. When the ignition coil malfunctions, it can lead to various issues that affect your car's overall performance. In this article, we will discuss the common symptoms of a faulty car ignition coil and how you can diagnose them. By understanding these symptoms, you can take appropriate measures to ensure the longevity of your ignition coil and prevent further damage to your vehicle.

Symptom 1: Engine Misfires

One of the most common signs of a faulty ignition coil is engine misfires. If you notice that your engine is running rough or sputtering during acceleration, it could be due to a malfunctioning ignition coil. Engine misfires occur when the coil fails to provide a consistent and strong spark to ignite the fuel mixture. A failing ignition coil can cause intermittent or permanent misfires, leading to a decrease in engine performance and fuel efficiency.

Symptom 2: Difficulty Starting the Engine

A faulty ignition coil can also make it difficult to start your car. If you find yourself struggling to turn the engine over, it could be a sign that the ignition coil is not generating enough spark. As the coil deteriorates, the spark produced becomes weaker, making it harder for the fuel mixture to ignite and start the engine. This issue is particularly noticeable during cold starts or after your car has been parked for an extended period.

Symptom 3: Decreased Fuel Efficiency

A failing ignition coil can significantly impact your car's fuel efficiency. When the coil fails to deliver a strong spark, it leads to incomplete combustion, resulting in wasted fuel. This inefficiency can cause a decrease in your car's mileage, and you may find yourself visiting the gas station more frequently than before. If you notice a sudden drop in fuel efficiency, it's essential to have your ignition coil inspected and potentially replaced.

Symptom 4: Stalling or Rough Idling

A failing ignition coil can also cause your car to stall or experience rough idling. When the coil is unable to provide a consistent spark, the engine may struggle to maintain a smooth idle or may even stall while driving. If you sense unusual vibrations or a fluctuation in the engine's RPM while at a stop, it is crucial to have your ignition coil checked. Ignoring this symptom can lead to more severe engine issues and costly repairs.

Symptom 5: Illuminated Check Engine Light

A malfunctioning ignition coil can trigger your car's check engine light. The engine control unit (ECU) constantly monitors the performance of various engine components, including the ignition coil. When a fault is detected, the ECU generates a trouble code, illuminating the check engine light on your dashboard. By using an OBD-II scanner, you can retrieve the trouble code and determine if the issue is related to the ignition coil.

How to Diagnose a Faulty Car Ignition Coil

Now that you're familiar with the common symptoms of a faulty ignition coil, let's discuss how you can diagnose the issue. Keep in mind that while these diagnostic methods can provide useful insights, it is always recommended to consult a professional mechanic for an accurate diagnosis.

Diagnostic Method 1: Visual Inspection

The first step in diagnosing a faulty ignition coil is to visually inspect it. Begin by locating the ignition coil. In most cars, you can find it near the engine, typically connected to the spark plug wires. Look for any signs of physical damage, such as cracks, burns, or corrosion. If the coil appears damaged or worn out, it's highly likely that it needs to be replaced.

Diagnostic Method 2: Spark Tester

A spark tester is a handy tool to determine if your ignition coil is generating a strong and consistent spark. Disconnect the ignition coil's primary wire and connect the spark tester in its place. Start the engine and observe the spark tester for a blue, consistent spark. If you notice a weak or intermittent spark, it indicates a faulty ignition coil.

Diagnostic Method 3: Resistance Testing

To perform resistance testing, you will need a digital multimeter. Begin by disconnecting the ignition coil from the engine. Set the multimeter to the ohms setting and measure the resistance across the coil's primary and secondary terminals. Consult your vehicle's service manual for the recommended resistance values. If the measured resistance deviates significantly from the specified range, it means that the ignition coil is faulty and needs to be replaced.

Diagnostic Method 4: Coil Swap

If you have access to multiple ignition coils of the same type, you can perform a coil swap to diagnose the issue further. Start by removing the suspected faulty ignition coil from your car. Replace it with the working ignition coil and start the engine. If the symptoms disappear or improve significantly, it confirms that the original coil was faulty and needs to be replaced.

Diagnostic Method 5: Professional Assistance

If you are uncertain about performing a DIY diagnosis or lack the necessary tools, it is always recommended to seek professional assistance. A qualified mechanic will have the expertise and specialized equipment to diagnose the issue accurately. They can perform advanced tests, such as an oscilloscope waveform analysis, to identify any underlying ignition coil problems that may not be evident through basic diagnostic methods.


The ignition coil plays a vital role in powering your car's engine. Recognizing the common symptoms of a faulty ignition coil can help you address the issue promptly and prevent further damage to your vehicle. Keep an eye out for engine misfires, difficulty starting the engine, decreased fuel efficiency, stalling or rough idling, and an illuminated check engine light. By performing simple visual inspections and using diagnostic methods like spark testing and resistance testing, you can diagnose a faulty ignition coil. Remember, when in doubt, consult a professional mechanic to ensure an accurate diagnosis and appropriate repairs.

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