Optimum combustion of the air-fuel mixture in an engine is vital for smooth operation, adequate acceleration and a refined powertrain. The spark plug is used to ignite the air-fuel mixture with a spark induced by a large voltage. The ignition coil provides the correct voltage across the spark plugs because the 12-volt battery is incapable of producing this high-voltage spark. The low voltage current is accumulated by the ignition coil and discharged across the spark plug at the right ignition timing. Faulty ignition coils lead to several engine problems so it is important to know the signs of bad ignition coils.

How does an ignition coil work?

Modern petrol engines need ignition coils to create a high voltage at the spark plug to produce a spark that ignites the air-fuel mixture.

The main components of the ignition coil include the primary winding and secondary winding of electrical wires. The ignition works as a step-up transformer to induce a high voltage in the secondary windings produced by the low voltage in the primary windings. The voltage induced in the secondary coil depends on certain factors such as the voltage in the primary coil, the number of windings and the strength of the magnetic field.

Voltages can be stepped up from 300 -400 volts to around 40kV. The battery system alone is not capable of producing such high voltage requirements of the spark plug. There are two main types of ignition coils used in modern cars. The first is a single-spark coil that is installed directly above each spark plug. In this system, one ignition coil is provided for each cylinder.

In multi-spark ignition oils, several coils are placed in a single block to supply all the spark plugs with the required ignition voltage at the right time. These systems are governed by ECUs that send the ignition voltage to the particular cylinder. Older cars had a point set that mechanically connects the ignition coil voltage to the required spark plug governed by the position of the crankshaft. The point set had to often be recalibrated by sliding it in its shaft to provide the correct ignition timing.

Also Read How To Start A Car Without The Key

What are the signs of bad ignition coils?

Bad ignition coils cause a range of problems from loss of power to poor fuel mileage. You should be aware of the symptoms of bad ignition coils so that you can have them replaced or repaired as soon as they start causing issues with the engine. Bad ignition coil symptoms in a motorcycle are also included here.

The following signs of bad ignition coils indicate a fault with the ignition system.

Drop in engine performance

If your ignition coils are giving trouble, you will immediately notice decreased engine performance and acceleration loss. Driving with a bad ignition coil will make the car sluggish and it will be slow to respond to throttle inputs. You may have trouble driving up an incline or keeping up with traffic on a highway. The problem lies with the ignition coil being unable to provide a strong spark to the spark plug. The required combustion will not occur and the engine will not produce enough power in all the cylinders. Some cylinders may stop firing and the fuel will go unburnt through the combustion chamber.

Also Checkout What Is A Fuel Injection System? And It’s Purpose

Dashboard warning lights

Modern engines are equipped with an onboard diagnostic device connected to the ECU (Engine Control Unit). Any issues with the ignition system including the ignition coils will send a signal to the ECU that reports this problem via the dashboard warning light.

There are several issues that dashboard warning lights will indicate such as checking the engine. These warnings are triggered by misfires in the cylinder through sensors, failure to start the engine and unstable engine operation. Failure to address the issue as soon as it crops up could lead to damage to the cylinders, electrical system or exhaust system. You should take your car to a service technician if you notice a check engine light warning on your instrument cluster.

Also Read The Difference Between A 2-stroke And 4-stroke Engines

Low fuel mileage

If you notice that you are filling fuel more often or that your fuel gauge seems to indicate low fuel after traveling a short distance, it could mean that a bad ignition coil is causing a drop in fuel economy. Basically, when the spark plug receives inadequate voltage from the ignition coil, it leads to incomplete combustion of the air-fuel mixture. The ECU compensates for this by adding more fuel to the cylinders through the fuel injectors leading to the wastage of petrol. Oxygen sensors in the exhaust system also record incorrect readings and tell the ECU that more fuel is required. This can also lead to raw fuel ending up in the catalytic converter of the exhaust system. If not rectified soon, this may lead to expensive repairs.

Must Read What Are Basic Bad Spark Plug Symptoms In A Car?

Exhaust backfiring

A bad ignition coil will cause the spark plug to fire poorly or the timing may go off. When this happens, the spark may ignite before the cylinder finishes its compression stroke causing backfiring. Unused fuel in the combustion chamber may also ignite without a spark due to the heat and pressure inside the cylinder. When the power stroke occurs at the wrong time, you will hear a loud bang from the engine which may be followed by loss of power and acceleration. It may also damage engine components and sensors. If you inspect the exhaust system, you will most likely see black smoke emitting from the pipes and notice the strong smell of burnt fuel. Have a technician check out the ignition system for a faulty ignition coil before you receive expensive repairs.

Also read Fuel System Cleaning: Clean the System Quickly

Difficulty starting the engine

The ignition system is sensitive enough to send the voltage at the precise time the spark plug requires it to burn fuel. Repeated ignition coil failure will not induce a spark at the right time especially when you try to start the engine. This may require you to keep cranking it till it turns over known as hard starting. The engine is cold when starting in the morning and the spark plug requires a high voltage to get the combustion process started. Malfunctioning ignition coils will cause trouble starting when the engine is cold. It is also possible that the engine may stall when you slow down at a traffic light or junction. At this point, it may refuse to start due to the ignition coil sending irregular voltage to the spark plugs.

Also Checkout Why do car engines overheat and how to deal with them?

Rough idling

You may notice the car moves in a jerky manner and the engine sounds rough while idling. It will sound as if the engine is running out of breath when the ignition coils are having a problem. The RPMs may vary without throttle input or may suddenly drop when you try to accelerate. Basically, the engine will not run smoothly as you feel it struggling to keep a normal pace. Bad ignition coil sounds you may hear include spluttering reminiscent of coughing when driving along. Due to the faulty ignition coil, one or more cylinders may not fire at all causing all these issues.

car accessories image

Vibrations from the engine

Even if your engine generally runs quietly without any drama on a usual day, a bad ignition coil may cause it to vibrate while the engine is idling. The cylinders are finely tuned to balance each other out and negate any harsh vibrations from the engine block. The crankshaft also has a counterbalance to prevent it from jerking. Ignition coil issues will cause cylinders to fire out of order, thereby going off balance and giving you the feeling that there is some slack in one of the engine components.

If you experience any of the symptoms mentioned above and suspect an ignition coil issue, have a certified technician check the electrical and ignition system thoroughly.

It could make the difference between a hassle-free ride and getting stranded in the middle of the highway. The sooner you address an ignition coil problem, the less chance of an expensive and lengthy repair.

Also Read Car Suspension – Working and Different Types Explained

How to test an ignition coil?

Some basic testing can be carried out to test the efficiency of an ignition coil. If you have trouble starting your engine, poor performance, misfiring, stalling or decreased fuel economy, it is advisable to test your ignition coils to pinpoint the problem.

Here are some methods you can follow to test an ignition coil.

Swap out the bad coil with a new one

If you replace the bad ignition coil and the engine symptoms like misfiring and stalling disappear, then you know that you had a faulty ignition coil. Start the engine and run it for a few minutes and you should find it running smoothly without any vibration. If you had a check engine light warning, even that should stop blinking.

Inspect the ignition coil

Look for signs of wear and tear such as cracks, melted plastic, burns or leakage from the ignition coil. If the ignition coil seems to be in a bad shape, it probably isn’t going to work well because electrical components are quite sensitive and will stir up a problem if they are damaged.

Use a multimeter to check the windings

The primary and secondary winding in the ignition coil should have a standard value of resistance. Use a multi-meter to measure this resistance and ensure it is within bounds. If the reading shows irregular resistance values, you know the ignition coil needs to be replaced.

Also Read All About Wheel Bearings + Faqs


Diagnosing signs of a bad ignition coil can sometimes be difficult for the uninitiated. Identifying symptoms is made easier when you know what to look for like poor engine performance, stalling, hard starting, misfiring, and decreased fuel economy and jerky driving.

Hopefully, this guide will help you find a faulty ignition coil and replace it in time. If all else fails, you can always have your car looked at by a professional mechanic to repair a bad ignition coil.

Have a look at some more interesting articles on the Carorbis blog such as the How Do You Know When Your CV Axles Are Going Bad? and Symptoms Of A Bad Throttle Position Sensor.

Frequently Asked Questions

Q1. How long can I drive my car with a bad ignition coil?

Ans. You could drive with a faulty ignition coil, but it is not advised. The ignition coil is responsible for supplying the voltage to produce a spark in the combustion chamber via the spark plug. A bad ignition coil will not produce an adequate spark, leading to incomplete combustion of the air-fuel mixture in the combustion chamber. The results could be misfiring of cylinders, backfiring in the exhaust system, loss of power, poor acceleration and low fuel mileage. Driving with a bad ignition coil can damage other critical components of the engine’s electrical system as well. Avoid driving the car on a failing ignition coil for more than 100 km. It will unnecessarily stress the engine by running on fewer cylinders. Modern cars have ECU systems that will stop the petrol supply to the affected cylinder, causing raw fuel to end up in the exhaust system. Have the ignition coil replaced as soon as possible for the smooth running of the engine.

Q2. Overheating ignition coil symptoms?

Ans. An overheating ignition coil will lead to failure eventually because the electrical system within the coils will be damaged by heat. The engine will subsequently experience problems with normal operation. The cylinders may begin to misfire, the engine will idle roughly, drop in power and acceleration, decreased fuel mileage, and the engine will hesitate to start, (if your car has an ECU, you may see a check engine light on your dashboard), backfiring in the exhaust system and unburnt fuel smell. If the ignition coils overheat, they will not be able to produce s strong spark which will disrupt the internal combustion process.

Q3. What are ignition coil failure causes?

Ans. Some of the most common causes of a failing ignition coil system are due to damaged or worn-out spark plugs. When spark plugs wear out, they draw more voltage from the ignition coils making them work harder and providing an additional strain on them. Engine vibrations can cause ignition coil windings and insulation to break or get damaged. Increased heat will decrease electrical conductivity, hindering the ignition coils from supplying the correct voltage and current. Wear and tear through heat and vibrations can cause ignition coils’ failure.

Similar Posts