how to know if ignition coil is bad

by:Haiyan     2023-06-30

How to Know If Ignition Coil Is Bad


The ignition coil is an essential component of your vehicle's ignition system. Its primary function is to generate the high voltage needed to create a spark in the spark plugs, which ultimately ignites the air-fuel mixture in the engine cylinders. However, like any other part, ignition coils can go bad over time due to wear and tear or electrical failure. In this article, we will explore the signs that indicate a faulty ignition coil, troubleshooting methods, and tips for replacement.

Symptoms of a Bad Ignition Coil

1. Difficulty Starting the Engine

One of the telltale signs of a faulty ignition coil is difficulty starting your vehicle. If you notice that your engine takes longer to start than usual or cranks but fails to start altogether, it might indicate a problem with the ignition coil. A weak or inconsistent spark can prevent the combustion process from initiating, making it harder for the engine to start.

2. Engine Misfires

When an ignition coil starts to fail, it can cause engine misfires. Engine misfires occur when the fuel-air mixture in one or more cylinders fails to ignite properly. You may experience a noticeable decrease in engine performance, rough idling, or a lack of power during acceleration. If left unresolved, constant misfires can lead to more severe engine damage, so it is crucial to address the issue promptly.

3. Stalling or Rough Running

A bad ignition coil can cause intermittent stalling or rough running while the engine is at idle or driving at low speeds. Stalling occurs when the ignition coil fails to provide a consistent spark to keep the engine running smoothly. If you notice persistent stalling or your vehicle feels rough when driving, it's wise to check the ignition coil for any potential problems.

4. Decreased Fuel Economy

A malfunctioning ignition coil can harm your vehicle's fuel efficiency. When the coil is unable to produce a strong spark, the engine may compensate by burning more fuel to maintain performance. As a result, you may notice a significant decrease in your vehicle's fuel economy. If you find yourself frequently refueling, despite normal driving conditions, a faulty ignition coil could be the culprit.

5. Check Engine Light Illumination

Modern vehicles are equipped with onboard diagnostic systems that monitor various components, including the ignition system. When a fault is detected, the check engine light will illuminate on your dashboard. While a lit check engine light can indicate various issues, a faulty ignition coil is one potential cause. If your check engine light turns on, it's advisable to have your vehicle inspected by a qualified mechanic to determine the exact problem.

Troubleshooting and Replacement

If you've identified one or more symptoms indicating a bad ignition coil, it's important to troubleshoot the issue and carry out the necessary repairs. Here are some steps to consider:

1. Perform a Visual Inspection

Begin by visually inspecting the ignition coil for any visible damage, such as cracks, corrosion, or loose connections. Check the wiring harness and connectors to ensure they are secure and free from damage. If any issues are found, replace the damaged parts.

2. Test the Coil with a Multimeter

To further diagnose a faulty ignition coil, you can use a multimeter to measure its primary and secondary resistance. Consult your vehicle's service manual or an online resource for the recommended resistance values for your specific ignition coil. If the readings are outside the acceptable range, it's likely that the coil is malfunctioning and needs to be replaced.

3. Replace the Ignition Coil

If the ignition coil is indeed faulty, it is best to replace it with a new one. Ignition coils can often be easily installed with basic hand tools, following manufacturer instructions or a professional mechanic's assistance.


Understanding the signs of a bad ignition coil is crucial for maintaining your vehicle's performance and preventing potential engine damage. By recognizing symptoms such as difficulty starting the engine, engine misfires, stalling or rough running, decreased fuel economy, or check engine light illumination, you can identify and address ignition coil problems early on. Remember to troubleshoot the issue and replace the faulty coil promptly to ensure the smooth operation of your vehicle's ignition system.

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