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Bad Coil Pack Symptoms and How To Test It

Bad Coil Pack Symptoms

A coil pack is a pack of ignition coils that is a part of a vehicle’s ignition system. It replaces the distributor on the new and modern vehicles since coil packs are more reliable than distributors. The coil pack is electronically controlled and is regulated by the car’s computer. There are several bad coil pack symptoms that you might encounter and you have to be aware of it as it could lead to a more serious engine performance issue. This is not surprising as a coil pack is responsible for building up the energy and taking the battery’s 12-volt output and converting it into as much as 45,000 volts and sends it through the spark plug cables which then gets to the spark plug to start the process of combustion. 

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The engine usually has at least four ignition coils, one for each engine cylinder's spark plug. An ignition coil plays a big role in the combustion process since without it, the spark plug won’t receive its needed voltage. Spark plugs need extremely high voltage to ignite the gas and air mixture in the combustion chamber. With it playing a significant role in starting the engine, one should know that whenever your car starts to give you trouble when starting or driving, it might be because of a faulty coil pack. Here are signs and symptoms of a bad coil pack:


  • Car Backfiring – One of the common bad coil symptoms is car backfiring. This occurs when the combustion takes place in the intake of the engine and not in the chamber. It causes a loud bang that is caused by an unexpected explosion in the internal combustion engine or at times, in the exhaust. A bad ignition coil can cause this since it can’t send the right amount of voltage to the spark plug which is needed to complete the combustion process. An incomplete combustion leaves unused fuel in the combustion cylinder that goes out through the exhaust pipe. There are times when the unused fuel explodes in the pipe before it can even go out. This will lead to a more serious damage. It is best to have it checked immediately when you notice that your car emits a black smoke or an obvious smell of gasoline or fire coming out from your exhaust. Delaying a repair could lead to greater damage and much more expensive problems. 



  • Poor Fuel Economy – When you have been driving your car for a while, you would know how much fuel it consumes on every trip. If you suddenly noticed that your car is consuming more fuel than your normal consumption, it could be a bad coil pack symptom. This happens when the system makes up with the spark plugs not getting enough power by feeding more fuel. The extra fuel consumption compensates for the lack of power given to the spark plugs. Poor fuel economy or poor gas mileage can also be caused by a faulty ignition coil. A wrong reading by the O2 sensor can cause the system to supply more fuel than what is actually needed. Having it checked and fixed can save you a lot of expenses later on.




  • Engine Misfiring –  One of a bad coil pack symptoms, engine misfiring can be heard as a coughing or sputtering noise or when you feel jerking motions when you try to start your engine or when you’re idling somewhere. This occurs when one or more engine cylinders fire improperly or won’t even fire at all. This is caused by a bad coil not supplying enough voltage to the spark plugs. Engine misfires can happen anytime but it usually happens when your vehicle needs to exert more effort such as when accelerating. Engine misfirings can cause damage to other car parts such as the catalytic converter since misfires produce a lot of emission. A misfire allows raw fuel to pass through the catalytic converter and it can saturate its interior causing oxidation and the afterburner process to stop which causes a carbon buildup and it will eventually be clogged. It can also damage the sensors with it changing the fuel-to-air mixture that can slow down your car when you try to accelerate. Misfires shouldn’t be taken lightly and the cause should be fixed as soon as possible.




  • A Noticeable Drop of RPMs During Acceleration – When you have a bad coil pack, you will notice a very obvious drop of RPM when you accelerate. You will notice this by checking the RPM gauge when you drive. A bad coil pack causes a serious lack of engine power.




  • Car Not Starting – If your car is not starting, it might be one of the bad coil pack symptoms. Multiple damaged ignition coils in a coil pack don’t produce spark which causes your engine to not start. The voltage needed to ignite the spark plugs is not met even though the starter motor is turned. With the missing ignition in the combustion process, the engine will fail to start, This problem is common in the cold season and when the humidity is high. 




  • Car Stalling – When your car stalls when you are driving or even when you are already stopped, the culprit could be a bad coil pack. At the early stage of a faulty coil, it delivers unsteady electrical currents to the spark plug. For this reason, you can still manage to start your car and drive it before experiencing a stall. When the stalling starts while you’re driving, your car might eventually stop completely and it will be harder for you to start the car again. 




  • Cracked Coil Pack – One way to determine a weak coil pack is by observing it. A coil pack is usually shaped like a box with ignition coils assigned to each cylinder. The pack and coils are exposed to the heat every time you’re driving your car. You can check on it constantly and look out for visible damages or cracks and even for loose coil connectors. It is best to have the coil pack replaced when you see cracks and not wait for another bad coil pack symptoms. It is always better to react early than to suffer later.




  • Check Engine Light Is On – The engine light in your dash will be illuminated when your car is having some engine performance issues. When you see your engine light turned on, it could be from a faulty coil. You can have it checked by a mechanic or yourself by using an OBD-II scanner. You can check for the standard Diagnostic Trouble Codes or the Engine Light Trouble Codes  in the scanner to find out what causes the problem. Coil-related problems have a code of P0350 to P0362. 



Can I Test a Coil Pack?

With the bad coil pack symptoms listed above, you might have encountered or experienced some of those symptoms and would like to check on your coil pack for further diagnosis. Fortunately, you can test the ignition coil pack yourself to find out what has caused your coil pack to fail.  


  • One thing you can do is by checking your coil pack and each ignition coil and examine if it has any visible cracks, corrosion, burn marks, and carbon tracking. Check the ignition coil’s housing if it’s in a perfect condition and make sure that there’s no oil leaks. Also inspect the ignition system’s wires for possible signs of burns and damage.


  • Measuring the coil’s primary and secondary resistance can also determine if it is functioning as it's supposed to. You can use an ohmmeter to measure it. You can begin measuring its secondary resistance by connecting the ohmmeter to the high voltage output terminal and the positive (+) terminal. A reading between 6,000 and 8,000 ohms of a new coil is normal while the others might go more than 15,000 ohms. Although ohmmeter are becoming less popular nowadays since the results it gives is not quite certain because the testing is done while the car rests and not when the coil is hot or under a load. This can cause the weak and bad coil to pass the resistance test.


  • You can also use a scanner to test the ignition coils. With a coil pack having multiple coils for each cylinder, you can switch or swap the suspected faulty coil to another cylinder and check it for possible misfires. If a scanner is not available, you can do a power balance test to cancel out each cylinder. To do this, you will have to listen for the RPM drop as you cut out each cylinder. The cylinder that doesn’t have much difference when cut out is the faulty one and is not contributing much to the vehicle’s RPM. Since other mechanical problems and fuel delivery can also cause the RPM to drop, to eliminate such possibilities, you will have to transfer the coil to a different cylinder and do the power balance test again. If the output is still the same, get rid of the faulty coil.


  • There is also a tool called a coil-on-plug (COP) probe. This tool makes use of a magnetic induction to check if the coil is firing. There is a probe that displays data on its screen and there’s one with a light that flashes to specify that the coil is working just fine. You can follow the instructions from its manual on how to use it properly. 


  • Another thing you can do is by using a spark tester. This tool needs to be installed in line with the ignition coil pack to determine whether the coil is producing spark or not. It would be better if you purchase or use a quality and adjustable spark tester since not all testers work with different types of ignition system. Be sure to use the right one. You might’ve heard of the process of removing the spark plug and grounding it rather than using a spark tester. While this can also ignite the spark but less voltage is used to fire a spark plug that has been removed from the engine compared to the one that’s still installed. It may not be enough to jump the plug gap with the engine running. Spark testers not only give the correct measurement or results in testing the coil pack on whether it is producing enough spark to jump the plug gap it is also safer than grounding the plug. 


  • Bad coil pack symptoms can be a little similar to other car part problems. That is why you have to be sure to keep you from misdiagnosis. One of the best tools you can use in diagnosing a coil pack problem is an oscilloscope. This tool displays the ignition system’s wave pattern. It measures the electrical signal through waveforms as it varies over time. The only downside of this tool is that its price is quite expensive. But it is always a good idea to invest in diagnostic tools for your car



The symptoms listed above are common types of bad coil pack symptoms. Knowing these symptoms can help you determine what kind of action you need to take to fix the problem of your car. When you experience one or more symptoms, having your car checked and fixed should be your top priority. Delaying it could lead to a more serious problem like a deteriorating engine performance and a damaged catalytic converter. Driving a car with a bad coil pack also compromises your safety as it tends to stall, misfire, and stop while you are driving. Remember that even if only one coil goes bad, chances are, the others will go bad too in no time. Fortunately, most ignition coils are not that pricey and are easy to replace. Having your car checked by a professional is a sure way to fix and get rid of the bad coil pack symptoms. If you are taking good care of your car and is up to date with its maintenance and services, then your car won’t cause you trouble any time soon.  

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