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7 Signs of a Bad Ignition Coil and Tips to Fix It is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for website owners to earn advertising fees by advertising and linking to Our endorsements are strongly based on our intensive research online and we do not accept sponsorships or gifts from brands in return. Learn more.

    A bad ignition coil, even though we do not think much about it when our car won’t start, have performance issues, or show a low fuel economy, is one of the major factors that directly affect the engine performance.

    When we experience some major problems with our car, like low fuel economy, engine backfires or having trouble starting, we usually check the spark plug first. The spark plug is, of course, one of the most important components that determine the overall performance of the car, and you should always use one of the best spark plugs available on the market. But if there is nothing wrong with the spark plug and the problem persists even after a spark plug replacement, the culprit could most probably be the ignition coil.

    Most of us are not aware of the ignition coil and why a perfectly functioning ignition coil is important, so we will explain all the things you should know about it, like what is an ignition coil, how does it work, what happens when it fails, etc.

    What is an ignition coil?

    The spark plug is what provides sparks to ignite the combustion, and the ignition coil is what draws current from the 12V battery for it. The 12V provided by the battery is not sufficient to create a spark good enough to ignite the engine, and so the ignition coil amplifies the 12V and provides the amplified current to the spark plug.

    A faulty coil will result in loss of power, a drop in fuel economy, and trouble when starting the engine. If you know how important the spark plug is for the engine, the ignition coil is equally important. The only difference is that it might not fail as frequently as the spark plug, and the signs of a bad ignition coil will be obvious.

    How does it work?

    The ignition system of a vehicle consists of an ignition switch battery, an alternator, an ignition coil, a spark plug, and a distributor. As mentioned earlier, the ignition coil amplifies 12V current thousands of times to provide the required voltage to ignite the fuel. Most cars use a 12V battery, and the voltage required to ignite the fuel is between 20,000V to 40,000V. In simple words, the ignition coil is a step-up transformer.

    The ignition coil has two windings, a primary coil wrapped around an iron core that accepts the input voltage and a secondary coil which has a very high number of turns compared to the primary coil. Just like a transformer, mutual induction causes amplification of the input signal, and the secondary coil transfers the high voltage to the spark plug.

    What are the symptoms of a bad ignition coil?

    A lot of problems could be pointing to a bad ignition coil, let’s see what are those.

    1. The car won’t start

    There can be a lot of reasons why your car won’t start, and a faulty ignition coil is one of them. A clogged or damaged spark plug, low battery, faulty alternator, imbalance in air-fuel mixture ratio, etc. could also result in the engine not starting.

    However, if you see more than two signs of a bad ignition coil, you should consider inspecting your car with a mechanic and replacing the ignition coil if necessary.

    2. The engine backfires

    We have covered all the information about engine backfires in a previous guide, mentioned that it can occur due to many reasons, and a faulty ignition coil is one of them.

    What causes engine backfire

    A backfire is a loud explosion taking place out of the combustion chamber of the car. Sometimes, it is accompanied by flames coming out of the exhaust, and sometimes it is just the noise. One of the reasons why an engine backfires is not having enough spark to ignite the fuel, which could result from a bad ignition coil.

    This issue is very dangerous and could cost a lot of money and precious time if not fixed immediately. While a bad ignition coil is not a huge issue if it is not causing many problems, an engine backfire is something that could damage the engine permanently. A simple ignition coil replacement might fix it now, later you might have to replace major engine parts.

    3. Poor fuel economy

    Just like the above two, poor fuel economy could be a result of a bad ignition coil. If you have been experiencing this along with starting issues and engine misfiring, you should have your coil checked. When the spark plugs are not receiving enough power, the engine compensates them by injecting more fuel into the combustion chamber.

    A poor fuel economy could result from a damaged or clogged spark plug, carburetor or fuel injection system issues, clogged air filter, etc. also.

    4. Engine misfires or stalls

    What could irritate us more if the engine chokes when we try to accelerate? If the performance of the car, the acceleration, or the power, decreases at higher rpm, probably your car has the problem we are talking about.

    An engine misfire happens when one of the cylinders fires incorrectly or doesn’t fire at all. It occurs mostly when we accelerate too fast, or while the car is idling at a stop sign, and it is accompanied by choking noises, vibrations, and jerking motions.

    When the engine misfires, it generates more emissions and can damage other parts like the catalytic converter as well. The sensors can get faulty, resulting in an uneven air-fuel mixture, which ultimately causes performance issues.

    5. Check engine light is ON

    The check engine light is there for a reason and when it turns ON, never hesitate to take your car to an experienced mechanic. It could be pointing to a lot of issues associated with the engine, but the bottom line is that something is wrong with your engine and you need to fix it as soon as possible.

    6. Oil leaks

    The coil housing has oil inside it, and it can overheat if the ignition coil is not functioning properly. It can cause the housing to break, and you will see an oil leak.

    If the spark plug is faulty, the ignition coil will operate at a higher voltage, which can cause heating and hence the oil leak.

    How to test bad ignition coil

    • Turn off your car and open the hood. Locate the ignition coil. The location will differ depending on the vehicle model, but it should be nearer to the wires connecting to the spark plugs. A simple search on the internet with your vehicle model number will help you locate it correctly.
    • Gently pull the wire connecting to one of the spark plugs.
    • Remove the spark plug, preferably by using a spark plug socket. Clean it using a soft cloth, especially if there are carbon deposits.
    • Attach the wire back to the spark plug.
    • Keep the threaded portion of the spark plug very close to a metal part. Note that you should not hold the spark plug in hand.
    • Remove the fuel pump relay.
    • Ask someone to turn the ignition key on. If you see continuous blue sparks between the metal and the threaded portion of the spark plug, your ignition coil is working fine. If it is not continuous, it could be faulty.

    How to prevent ignition coil from going bad?

    Most of the problems are associated with the ignition coil working at a higher voltage to compensate for a damaged spark plug, and hence keeping your spark plug clean and healthy always is the key. Frequently checking the spark plug gap and cleaning the carbon deposits will ensure good performance and a great fuel economy, as well as keeping your car away from having a bad ignition coil.