Symptoms Of A Bad Coil Pack And How To Test

The ignition coil is an important part of a modern electronic ignition system. Knowing the symptoms of a bad coil pack is important in understanding what is wrong with your vehicle.

The ignition system needs to complete three things for your vehicle to run properly. It must deliver a spark to the engine that is strong enough, at the correct time, and long enough to correctly ignite the air fuel mixture. 

The ignition coils role is to supply the ignition components with a high voltage for the spark plug to “fire”. While modern ignition coils are much more reliable than older ignition systems, they can still fail. Let’s take a look at the symptoms of a bad coil pack and see how we can get your vehicle running well again. 

The ignition system of a vehicle is split in two different sections, they are called the primary ignition components and the secondary ignition components. It is important to know the differences and the role that the ignition coil pack plays. 

Ignition System Components

Primary Ignition Circuit Components

The primary ignition circuit consists of a few vital components that allow power to reach the ignition coil. 


Like many other systems in your vehicle the battery provides power to the ignition system.

Triggering Device

This component is generally a position sensor on the engine. The crankshaft position sensor or camshaft position sensor are generally used by electronic ignition systems. The position sensor provides a signal to the engine control module so it can determine the position of the piston in each cylinder and determine when to fire the spark plugs. 

Switching Device

The switching device is what actually turns the primary coil of the ignition coil on and off. The PCM is used by most modern vehicles as the switching device. 

Ignition Coil

The primary windings inside of the coil is the final part of the primary ignition circuit components. 

image of an ignition coil pack under the hood of a vehicle

Secondary Ignition Circuit Components

Ignition Coil

The ignition coil pack is an interesting component of the ignition system as it is part of the primary and secondary ignition system. The secondary windings are part of the secondary ignition system. 

Distributor Cap and Rotor

Yes, a distributor can be part of an electronic ignition system, they are not as common on modern vehicles. The distributor’s role is to distribute the spark to the correct spark plug wire at the correct time. Most modern vehicles are not equipped with a distributor. Instead a coil-on-plug sits on the spark plug and eliminates this component. 

Spark Plug and Spark Plug Wires

close up image of spark plug wires installed on an engine

The wires are what provide a path for the electrical charge to travel from the ignition coil to the spark plug.

This is the end of the ignition system. The electrical charge is sent to the spark plug where it jumps the gap and provides a strong spark to ignite the air/fuel mixture that is present in the cylinder.

How Does A Coil Pack Work?

An ignition coil pack is a very cool piece of automotive technology. It works with the principles of magnetism and electromagnetic induction. Yes, I know those are some very big words being thrown around. Do not worry, I will keep it simple to help understand exactly what is going on inside of the ignition coil. 

In the most simple of terms, the coil pack acts as an electrical transformer. The spark plugs require 12,000-20,000 volts to jump the spark plug gap and ignite the air fuel mixture that is in the cylinder. They take the 12 volts that is supplied by the engine and supply the high voltage of up to 20,000 volts to the ignition system. As simple as it is, the role the ignition coil plays in the ignition system is very important. 

As you read above the coil pack has two separate windings, the primary and secondary windings. When voltage is supplied to the primary windings they create a magnetic field. 

When voltage is removed from the circuit the magnetic field collapses. Because both the primary and secondary windings are inside of the coil pack. The collapsing of the magnetic field causes voltage to be induced into the secondary winding of the ignition coil and increases its strength. The electrical charge then travels through the spark plug cables to the spark plug where the spark jumps the spark plug gap.

Symptoms Of A Bad Coil Pack

There are many different things that can be present when you have symptoms of a bad coil pack. Below are the top 8 symptoms that you may notice as you are driving. 

Poor Fuel Economy

If you have noticed a large drop in your fuel economy lately, a faulty ignition coil may be the cause. Simply put, a bad ignition coil will result in poor fuel economy. This is caused by the fuel that is injected into the engine not burning fully. When this happens the engine will inject more fuel into the engine and cause the fuel economy to suffer. 

If you are having poor fuel economy take a look to see if there is smoke coming from your tailpipe. Black smoke coming from the tailpipe is a sure sign that fuel is not being burned in the engine. 

Loss Of Power

Is your vehicle not accelerating when merging with traffic?  This could be caused by a faulty ignition coil or bad spark plugs. A loss of power may be noticed along with poor fuel economy. 


Bad ignition coils can cause your vehicle to backfire when you are driving down the road. A backfire is caused when there is unburned fuel in the exhaust pipe that ignites. The explosion of the fuel igniting is what causes the backfiring sound. 

A bad ignition coil or spark plugs could cause the fuel in the cylinder to not burn. When this fuel is sent through the exhaust system the backfire occurs. 


When your vehicle feels like it is jerking or hesitating when accelerating it can mean that there is a bad ignition coil. When this symptom occurs you will also likely notice that the vehicle has less power than normal, or a possible engine misfire.

Engine Misfire

There are many reasons that an engine misfire can occur such as a lean mixture, incorrect timing, faulty ignition coil, etc. 

Engine misfiring occurs when the air/fuel in the cylinder does not ignite. This symptom can be caused by a faulty ignition coil or possible spark plugs. 

Modern vehicles will pick up the engine misfires and illuminate the check engine light. 


Spark that is being delivered to the engine at the wrong time because a bad ignition coil can cause the engine to stall. Stalling can occur when the vehicle is idling or worse yet, when you are driving down the road. 

Stalling is more common on vehicles that have one individual ignition coil that provides the electrical charge to all of the cylinders. 

A vehicle with coil on plug ignition(one coil per spark plug) is not as common for this symptom to be present. 

Check Engine Light 

One of the most common symptoms of a bad ignition coil is the check engine light illuminating. Modern vehicles have great self diagnostics that they can pick up the bad ignition coils very quickly. 

If your check engine light is illuminated, get the code checked to see what is wrong with your vehicle. 

If your check engine light is flashing, make sure you stop the vehicle when it is safe to do so. A flashing check engine light means that there is a catalytic converter damaging event and most likely the engine misfires. This means if you drive with the light flashing you can cause damage to the vehicle’s catalytic converter. 

Hard Starting

This is going to be more noticeable when you are first starting the vehicle or the vehicle is cold.

If the ignition coil is not providing spark to the engine it will not start or if the spark occurs at the wrong time, it may cause the engine to have an extended crank and be hard to start. 

How To Test A Coil Pack

Tools Needed To Diagnose A Bad Coil Pack

A spark tester that is used for testing HEI ignition systems and ignition coils

Testing an ignition coil requires some specialized tools such as a test light, automotive multimeter and a spark tester. Be sure to know how to use a test light before starting the repair. With these tools there are some tests that you can perform to see if you have a bad ignition coil.

The testing that you can do is limited to the type of ignition coil that your vehicle is equipped with. 

Testing For Spark

I would suggest that this is the first test that you do when diagnosing a bad ignition coil. 

A spark tester attached to an engine ground ready for coil pack testing
  1. Remove the spark plug wire from the spark plugs (be careful when you remove stuck on spark plug boots)
  2. Install the spark plug tester
  3. Have a helper crank the engine while you observe the tester
  4. If there is a bright, blue spark present, the ignition coil is working
  5. If there is no spark present check the wiring to the ignition coil.

Before you replace the bad ignition coil make sure that the wiring to the ignition coil is okay. Here are a few simple tests that you can follow below to check the wiring. 

Coil On Plug Testing

Coil-on plug ignition coils are unique as each cylinder has its own ignition coil. The tests that you can perform on this type of ignition coils are related to wiring. 

Remove the connector and complete the following tests. 

  1. Test the feed circuit for proper battery voltage
  2. Probe and test the ground circuit
  3. Compare readings with specifications. 

Multiple Cylinder Ignition Coil Testing

The ignition coil that your vehicle is equipped with will dictate what you can test. The coil in the images is from a Dodge Grand Caravan and allows us to test the secondary circuit of the coil. 

Remove the wiring connector and complete the following tests:

  1. Check the feed circuit for proper battery voltage
  2. Test the ground circuit
  3. Measure the resistance of the secondary ignition circuit. This can be done by probing the secondary coil terminals with an automotive multimeter (see image)
A bad coil pack that is being test for resistance with an automotive multimeter

4. Compare readings with specifications. 

An ignition coil is a complex electrical component and requires specifications to be met. If your readings are outside of the factory specifications, replace the ignition coil. 

Ignition Coil Replacement Costs

Replacing an ignition coil is a great do-it-yourself repair for people of all skill levels and depending on the symptom of a bad coil pack you will need to replace it. Depending on the manufacturer and engine that your vehicle is equipped with and if the vehicle is equipped with coil on plug or a different type of ignition system. Expect an ignition coil to cost between $75 and $200 to buy the part and replace it yourself.  

If you do not have time or are not comfortable doing this repair. A professional mechanic would charge between $200-$400 to replace the ignition coil. 

If you have a bad ignition coil replaced, I would recommend that you get the spark plugs and spark plugs wires replaced at the same time. 

While having bad ignition coils can have drastic issues while driving the vehicle, knowing what the symptoms of a bad coil pack are can ensure your vehicle is running in the best possible manner. There is nothing worse than having a bad coil pack and burning more fuel than you need today. Especially with today’s fuel prices.

John Morris
John Morris
John Morris is the technical editor for His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

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