Symptoms & Replacement Of A Bad Ignition Coil

5 Signs Your Ignition Coils Are Failing And How You Can Replace Them

The three main prerequisites for the engine to run are fuel, air and spark. Often when a vehicle is unable to cold start or has trouble idling, a common step is to replace the spark plugs. However, the spark plugs are not always at fault, especially in new Coil on Plug (COP) ignition systems where there is an ignition coil for every spark plug.  Unlike older systems where there was just one coil for all plugs, in COP systems when one ignition coil fails it does not affect the rest of the coils or plugs. This means you could have a single spark plug not firing because of a bad ignition coil. This is why it is important to know the signs of failing ignition coils and how to replace them. 

In this article:

What Are Ignition Coils How Long Do They Last?

Ignition coils work on the principle of electromagnetic induction and are essentially a step-up transformer that converts the battery’s 12 volts to over 40 thousand volts necessary to create a spark that lights the fuel and air mixture. Some vehicles have a single ignition coil that provides a spark to all the cylinders, however, most modern vehicles have one coil for every cylinder. This system is called Coil on Plug (COP) system wherein the ignition coil sits right on top of the spark plug in the cylinder head and eliminates the use of distributors as found in older vehicles with single ignition coils. 

Although modern COP systems are far more reliable than mechanical distributor systems, they are prone to damage with prolonged use and will require at least one replacement in the vehicle’s lifetime. They last anywhere from 100,000 km (~62K miles) to 160,000 km (~100K miles) depending on the vehicle and driving condition among other factors. 

If you are worried your vehicle has crossed that threshold and suspect you have worn out ignition coils then take a look at the symptoms of a failing ignition coil that can help you determine if you need to replace your ignition coils

What Are The Signs Of A Bad Ignition Coil?

Failing ignition coils can have many adverse effects on the engine and the following are a few signs that you will likely notice:

Check Engine Light

The first indication of ignition coil failure is displayed by the check engine light. The ignition coils are major components in the ignition system and without properly functioning coils the engine is likely to suffer, resulting in the ECU sending out error codes that light up the check engine light. The common error codes received when checked with a vehicle diagnostic tool are P030X (where X = cylinder number) or P0351/P0350.


This occurs when fuel leaves the combustion chamber without undergoing combustion and ignites in the exhaust system causing a backfire. This is due to the lack of necessary spark to ignite the fuel-air mixture as a result of broken ignition coils or spark plugs. You may even smell gasoline coming from the exhaust pipes. This is especially disadvantageous since it can cause catalytic converter failure with excess unburnt hydrocarbons in the exhaust system.

Loss Of Power And Fuel Economy

Failing to create a spark and igniting the fuel in the combustion chamber means the cylinder will not undergo a power stroke or simply put, the cylinder won’t produce any power. If one or more of the coils are faulty then you can expect a steep decline in the fuel economy along with loss of engine performance that becomes more evident while accelerating.

Unable To Start The Car

Older vehicles with single ignition coils can face the issue of complete stalling if the ignition coil is broken as this disables sparking in any of the cylinders. COP systems can also experience this issue, especially if more than one ignition coil is damaged.

Engine Misfires

Faulty ignition coils cause intermittent sparking that results in misfiring of the engine and at higher speeds, the engine can cough or sputter. Irregular idling, occasional engine stalling and increased vibrations can be another indication that the ignition system is failing to do its job. This system can be more prevalent on COP ignition systems since even if one of the ignition coils breaks, the engine will misfire.

If the ignition coils are damaged then it can prevent spark plugs from functioning efficiently and the same goes for spark plugs that are worn out or well overdue for a replacement. Damaged spark plugs can deteriorate the ignition coils by making them work harder for sufficient sparking, leading to more expensive fixes. Hence always make sure to replace the spark plugs along with the ignition coils. You can opt for platinum spark plugs or iridium spark plugs that last much longer and provide a trouble-free life. Here's a guide on choosing the right spark plugs for your vehicle. The next section will uncover the replacement job of the ignition coils and spark plugs.

How To Replace Faulty Ignition Coils?

The method of replacing the ignition coils depends on the type of ignition system. COP systems have ignition coils placed right on top of the spark plugs and sit in the cylinder head, making the replacement job solely dependent on cylinder head access. Single ignition coils are usually mounted away from the engine and on top of the distributor, whereas the DIS system with single-coil or a coil pack is situated near or around the engine. The replacement task for these systems is pretty straightforward.

Before you begin, have a look at the tools required to replace ignition coils and spark plugs:

Follow These Steps For A Coil On Plug System:

  • Wait for the engine to cool down. As a safety measure disconnect the negative battery terminal by undoing the nut holding the cable. 
  • Now remove any engine cover and locate the ignition coils. These are usually black plastic housings with wires running to them. For a better understanding, make sure to look up ignition coils in your vehicle service manual.
  • Ignition coils can be attached via bolts or simply snap into place. Remove any fasteners and proceed to remove the electrical connections.
  • Once the connections are off, pull the coil out. Note: Always remove only one connection and coil at a time so as to prevent mixing up the wires for different cylinders. 
  • Now that the ignition coil is removed you can access the spark plug. Insert the spark plug socket and using a ratchet, remove the spark plug. 
  • Situate the new spark plug in the cylinder head and tighten it using the socket and ratchet. Before you insert the ignition coil, apply a small amount of dielectric grease to the coil boot to make later access easier. Snap the coil in place and restore the electrical connection. Redo the bolt to the coil if required. 
  • Repeat this process one by one for every ignition coil and spark plug. 
  • Then secure the battery negative terminal cable in place and start the car. The check engine light shall disappear after two complete running cycles. 

Follow These Steps For Systems With Single Ignition Coil:

  • Wait for the engine to cool down first. Then disconnect the negative battery terminal cable. Then remove the air filter in carbureted engines or any engine cover and locate the ignition coil pack. Refer to your vehicle’s service manual for more information. 
  • Once you locate the coil, remove all the ignition wires attached to it. Make sure to remember the order or orientation of the wires leading to the coil pack. Mixing up the wires can lead to improper engine sparking and ultimately decreased performance. Use a tape and marker to note down the wires in order. 
  • Now proceed to remove the mounting bolts and any other connections, freeing the coil for removing.
  • Inspect the new and old coils to make sure they are an exact match. Then place it properly and bolt it back in place. 
  • Now one by one, remove the spark plug wire and then the plug using the socket and replace it with a new spark plug. Tighten the plug to correct torque spec. Never over-tighten it. Refer to the service manual for torque specifications. 
  • Restore the connections and attach the spark plug wires in their right order. 
  • Once all spark plugs are replaced, attach any previously removed components and the battery cable and proceed to start the car. 

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John Framigllia
Technical Writer
Our technical writer is known for simplifying automotive parts and services. Intuitive with various vehicles and manufacturers, he knows how to simplify complicated problems.