Skip to content

Spark Plug Replacement: 6 Common Bad Spark Plug Symptoms 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.

    Each engine component has to work perfectly for the smooth and seamless running of your car. As time passes, some components fail and as a result, the overall performance will be affected, ranging from engine misfires and stalling to poor fuel economy.

    A bad spark plug, if not maintained and cleaned properly, could get damaged or clogged by carbon deposits in about a year. Being the component that initiates the ignition and starts the engine, it is extremely important to check the spark plug frequently and keep it healthy all the time.

    When the spark plug has issues, whether it is damaged, clogged due to carbon deposits, or has an imbalanced gap, it will be evident from the performance of the car. The signs, if not diagnosed and fixed timely, could lead to bigger problems like a bad ignition coil, engine problems, etc.

    However, there is no clear sign which exclusively says that the spark plug is the culprit. The signs that we get could be symptoms of a bad spark plug, but it could also be due to damages to other components in the ignition system, like the ignition coil, sensors, fuel injector, etc. These signs tell us that there is something wrong with the ignition system and it could be due to a faulty spark plug.

    We will explain the bad spark plug symptoms below, and if your vehicle shows more than one sign, it is better that you take it to a mechanic and get it replaced. A spark plug wouldn’t cost much, the replacement is easy and can be done yourself.

    So, what happens when spark plugs go bad?!

    6 most common symptoms of bad spark plugs

    1. Engine misfires or idles often.

    Engine misfiring is one of the most common bad spark plug symptoms. You will feel like the engine is missing a beat or two at times, especially when you are idling. There will be small jerks and vibrations along with it. In short, you will feel like it’s not a smoother engine anymore.

    If the misfires are a little intense, your ‘check engine’ light will show up. The exhaust emissions will increase, and the fuel economy will drop. You will hear intermittent stumbling or sputtering sounds from the exhaust frequently.

    A spark plug replacement would be well worth the money spent if the engine is misfiring—you’ll see a significant improvement in your gas mileage. Besides, spark plugs are not expensive.

    2. Poor fuel economy.

    Poor fuel economy is the worst side of a bad spark plug. If you drive hundreds of miles a day, it could be costing you a lot of money.

    But why are we saying that the fuel economy will drop?

    The spark plug, as can be understood from its name, is what provides the spark to ignite the air-fuel mixture. When it is not working properly, there won’t be enough sparks to ignite the whole fuel in the combustion chamber, which results in some of the fuel escaping through the exhaust uncombusted.

    A poor fuel economy could result from a lot of other things, like a bad ignition coil, fuel injection system damages, etc. but the most common reason is a bad spark plug that needs replacement.

    3. The car fights to accelerate.

    Bad spark plug symptoms

    If you have been experiencing poor acceleration, jerking, or power loss while accelerating, it could be a bad spark plug symptom. Other issues such as faulty sensors, clogged fuel injector or carburetor, bad fuel filters, etc. can also cause acceleration issues. It is not an exclusive sign of a faulty spark plug, but it can be a bad spark plug symptom.

    Modern cars use multiple sensors that determine when to send electric pulses to the spark plug, so a faulty sensor can also result in this problem. In both cases, it will be accompanied by a poor fuel economy, and it is better to get it checked by a mechanic for a perfect diagnosis.

    4. The car has trouble starting.

    Another sign of a bad spark plug is an engine that hesitates to start. The initial combustion needs good, consistent sparks, which a faulty spark plug will not be able to provide. The engine may give you a very long cranking time before it starts.

    Again, just like other signs, this could be due to other issues in the ignition system also.

    5. Rough idling.

    Rough idle results from a lean or rich air-fuel mixture. A lean air-fuel mixture means it has more air than needed and does not have sufficient fuel, and a rich mixture means it has more fuel but no enough air to get it combusted. But in case of a bad spark plug, even if the mixture is well-balanced, there won’t be sufficient sparks for complete combustion, so the idle becomes rough.

    The amount of fuel is determined by sensors and the oxygen sensors will regulate the mixture. When the bad spark plug causes incomplete combustion, there will still be more fuel left—the oxygen sensors will get a faulty reading and that will result in even more misfires. It isn’t good for the engine in any case, and it is important to get it fixed before it damages other parts of the engine.

    A bad spark plug sounds like rough idling—and it is one of the most prominent symptoms of a failing spark plug.

    6. Engine backfiring.

    We have explained the causes of engine backfire before, and mentioned a bad spark plug as one of the possible reasons. A faulty spark plug will not ignite the fuel completely, which will result in some fuel escaping through the exhaust pipe. It will get burnt inside the exhaust due to heat and friction and we will hear the sound of an explosion, sometimes accompanied by fire coming out of the exhaust.

    Bad spark plug symptoms: FAQs

    What will happen if I keep driving with a bad spark plug?

    Well, if you keep driving with bad spark plugs, the consequences can vary, but in the long run, it may result in:

    1. Your catalytic converter getting damaged. The unburnt fuel will burn near the catalytic converter due to heat, which will eventually damage the catalytic converter. Replacements are very costly.

    2. The engine will keep running, keep misfiring and the spark plug will keep getting clogged, until a time when the engine does not start and the spark plug does not work.

    3. It can damage other parts of the engine due to high carbon deposits.

    How to maintain a spark plug in good condition?

    Periodic cleaning of spark plugs is necessary if you want them to last longer. A spark plug will last 4-6 years and 35000-55000 miles if maintained properly.

    What you have to do is to clean the spark plug using a soft cloth every 7000 miles, make sure that the gap between electrodes is as recommended by the manufacturer.

    You will notice the engine becoming smoother after cleaning the spark plug. The carbon deposits will clog the electrodes eventually causing a weakness in the spark. When we clean it, it will return to normal condition.

    How much does a spark plug replacement cost?

    It actually depends on the number of spark plugs your vehicle has, and also the engine model. If your vehicle has three twin-spark cylinders, there will be six spark plugs. Usually, a vehicle can have four to ten spark plugs. Your service manual or a simple search on the internet will give you the required details.

    A normal spark plug costs around $5, platinum around $8, and iridium around $9. Assuming that your vehicle has six spark plugs, the total cost for spark plugs, if you choose the best will be well below $60. Most spark plugs are easy to replace and takes only around 10-20 minutes. So the total cost to replace spark plugs will be around $160, usually in the range of $120-$140.

    Which spark plug to choose?

    There are different types of spark plugs available on the market, like iridium spark plugs, platinum spark plugs, and nickel-alloy spark plugs. I prefer Iridium and platinum spark plugs to normal ones since they provide a stronger spark, and also last longer than the normal ones. Of course, they are a little more expensive than normal spark plugs, but they will prove their worth in the long run.

    How do we know that iridium spark plugs are the best? It’s simple. When you replace the normal ones with iridium, your tachometer reading will go up, which indicates better combustion and hence better fuel economy and performance.

    NGK and Bosch are the best brands on the market, and if you’d like to have the best spark plug, read our review of the best 16 spark plugs on the market.

    Please note that going for the cheapest alternatives for the sake of money would do more harm than good. A bad spark plug can cause other damages to the ignition system, like ignition coils and cables, which costs hundreds of dollars to replace.

    What is the perfect spark plug gap?

    Too much spark plug gap can cause misfires

    In simple words, the perfect spark plug gap is what your manufacturer has mentioned in the service manual.

    The spark plug gap is fairly critical, which can directly affect the overall performance of the car, including fuel economy. Most manufacturers set it between 0.028″ and 0.060″, but it will vary according to the vehicle model.

    An improperly gapped spark plug is as bad as a faulty one and will show the same signs as a bad spark plug. The worst part is that it will damage the ignition coil since the ignition coil will have to work harder to create sparks and eventually you will have to replace the ignition coil.

    To learn more about spark plug gaps, you can read this article by NGK.

    Where are spark plugs located?

    Again, it depends on your vehicle model, but we can help you.

    On a general note, the spark plugs are located in the cylinder head above the combustion chamber. V engines usually have the cylinders in a V and the spark plugs can be found on the outside of the cylinder head. In some cars, it will be at the top of the valve covers.

    If it is an inline 4,5, or 6 engine, your spark plugs can be found at the top of the valve cover. In older cars, it might be fitted on the side of the cylinder head.

    The spark plugs are usually easily accessible since it is a part that needs replacement at times.

    How to replace a bad spark plug?

    Once you have located the spark plugs, it is easy.

    1. Make sure that the engine has cooled down.

    2. Use a socket and wrench to disconnect the negative terminal of the battery. It is always a good practice to disconnect the battery when working with the electrical systems.

    3. Remove the ignition cables by gently pulling it outwards.

    4. Carefully remove the spark plug using a socket or wrench by turning it counter-clockwise. Using a spark plug socket is recommended.

    5. Check if the size of the new spark plug matches with that of the bad spark plugs. If they do not match, get it replaced.

    6. Keep the new spark plug in place and rotate it clockwise using hand(if accessible by hand). Tightening it first using hand is necessary to avoid issues.

    7. Reconnect the ignition coils and cables.

    8. Connect the negative terminal back to the battery.

    9. Turn the ignition key on keeping the car in idle and gently press the accelerator to see if everything is working fine.

    Kudos! You have successfully replaced your spark plugs.


    The most important points about bad spark plugs and their replacement can be noted as:

    • Common signs of a bad spark plug include engine misfires, poor fuel economy, acceleration issues, hard to start, rough idle, and engine backfiring.
    • If you keep driving your car with a bad spark plug, it can damage other parts such as the catalytic converter and sensors.
    • Periodic cleaning of spark plugs can make them last longer and it will keep your car engine in good condition.
    • Iridium spark plugs are the best, platinum comes next. These spark plugs provide a better spark and last longer than normal spark plugs.
    • A spark plug replacement can cost around $150, depending on the spark plug you choose. On most cars, it will take around 20 minutes to replace them.
    • Normal spark plug gaps are between 0.028″ and 0.060″ and it is better to keep it as recommended by the manufacturer.

    Happy motoring!