Top Symptoms of Bad Ignition Coil (Causes and How To Test It)

Ignition coils, often subjected to heat, are susceptible to failure. When the ignition coil goes bad, it produces different symptoms.

Bad Ignition Coil Symptoms

The main function of the ignition coils is to transform the low-voltage current coming from the vehicle’s battery into a high-voltage current to spark the air-fuel mixture and help the engine to produce power. Any issues with the ignition coil can result in a spectrum of mechanical and performance-related problems.

This article will help you understand what happens when an ignition coil goes bad and explain how it works. We will also explain to you the location of the ignition coil and how to diagnose it if it’s working properly.  

What is the function of an Ignition Coil?

The ignition coil, also known as a spark coil, is a critical component in the ignition system of an internal combustion engine.

The main function of the ignition coil is to transform the low voltage (12V) power from the battery into the high voltage (thousands of volts). This high-voltage power is utilized to produce a spark at the spark plug, which ignites the air-fuel mixture within the engine cylinders.

Ignition Coil

The ignition coil contains two sets of coils wrapped around an iron core. In older engine models, a single ignition coil is common. In these engines, the output of the ignition coil is channeled to each cylinder through a distributor. Different small engines, like those in lawnmowers, still employ this arrangement.

Conversely, the latest automotive engines typically have a distributorless system, such as the coil-on-plug setup, where each cylinder possesses its own dedicated ignition coil.

Symptoms of A Bad Ignition Coil

When the ignition coil of your vehicle goes bad, the engine control module (ECM) activates the check engine light. The bad ignition coils may also lead to a stalling or misfiring engine, along with a rough idle, poor fuel economy, or rough acceleration. It may also lead to a hard-starting engine.

The following are the most common symptoms of a faulty ignition coil:

1) Check Engine Light

Check Engine Light, signs of bad ignition coil

The vehicle’s computer monitors the ignition system performance. It may turn on the check engine light if it detects an issue, such as a misfire or other ignition problems.

A diagnostic trouble code such as P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0309, P0310, P0311, or P0312 will also be stored in the computer’s memory.

If the check engine light is steadily illuminated, it may provide a window of time for you to fix the issue before it escalates. However, driving in this state can potentially lead to more severe damage. If the light is flashing, it’s a sign to immediately shut off the car to prevent further engine damage.

2) Engine Misfires

Engine Misfiring

When an ignition coil isn’t functioning properly, it results in an engine misfire, which happens when the air/fuel mixture in the combustion chamber fails to ignite as it should.

Engine misfires cause a noticeable stuttering or jerky sensation, particularly during acceleration. As the misfire intensifies, you may feel vibrations in your vehicle’s cabin.

3) Hard-Starting Engine

Hard Starting

Starting an engine requires a proper spark to ignite the air/fuel mixture at the correct moment. If your spark plug is unable to receive the right voltage from the ignition coils, the fuel will not ignite in the combustion chamber as it should.

You can easily observe this issue during the engine start-up process. If you’re experiencing difficulties starting the engine, it might be due to faulty ignition coils or other parts within the ignition system.

4) Rough Idle

Rough Idle, symptoms of a faulty ignition coil

Engine idling refers to a condition when your engine is operating, but the car isn’t moving. This is most common when you’re stopped at traffic signals or intersections or parked and waiting.

Generally, engine idling should be steady and consistent. If it starts to run roughly, it could be a sign of problems with the ignition coil. However, your engine may idle due to many other issues, such as a bad MAP sensor, bad camshaft sensor, bad spark plugs, faulty fuel injectors, faulty fuel filters, or faulty air filters.

Read More: Rough Idle Symptoms and Causes

5) Reduced Engine Power

Poor engine Power

A reduction in engine power is one of the early signs of a bad ignition coil. If your ignition coil is malfunctioning, your car may feel lethargic and show a noticeable decrease in power. This is due to irregular spark generation, resulting in poor combustion of the air-fuel mixture.

If you encounter a reduction in your engine power, it’s advisable to contact a professional for servicing to prevent additional damage that could result in more expensive repairs.

6) Poor Fuel Economy

Poor Fuel Economy

Ignition coils of your vehicle may become faulty all at once or progressively over time. If the coil is deteriorating over time, one clear sign would be a drop in fuel efficiency.

If you keep a close eye on your fuel mileage, you might spot issues even before other symptoms become apparent. Given that you want your engine to operate efficiently and avoid spending unnecessary money on gas, it’s advisable to change a faulty ignition coil promptly as it goes bad.

7) Stalling Engine

Stalling Idle

A stalling engine is one of the common symptoms of a bad ignition coil.

When the ignition coil of your vehicle can’t sustain the engine, it will stall due to improper combustion.

In extreme cases, the engine may not restart, leaving you stranded at your current location when the engine comes to a halt.

8) Exhaust Smells Like Gas

Gas smell due to bad ignition coils

The job of the ignition system is to ensure the proper ignition of the fuel-air mixture in each cylinder. If this system fails to do so, your engine will be subjected to an uneven flow of fuel and air.

This imbalance leads to a surplus of fuel being expelled via the exhaust. The fuel of your vehicle contains a unique smell, so the smell of unburnt fuel being released from the exhaust will be easily noticeable.

9) Spluttering and Coughing Noise

An issue with the ignition coils may also lead to sputtering and coughing noises during engine start-up. These noises originate from a cylinder that’s failing to fire during the ignition sequence.

Causes of A Bad Ignition Coil

The ignition coil may go bad due to one or more of the following reasons:

1) Wrong Installation

The wrong installation of the ignition coil (like inadequate securing or improper wiring) may force it to go bad early.

2) Vibrations

Excessive engine vibrations may harm the ignition coil windings and insulation, leading to breaks or shorts in the secondary windings.

3) Wear And Tear

Regular wear and tear is a common cause of ignition coil failure.

As they get older, the insulation between the different coil windings and within the primary coil starts to degrade. This deterioration in insulation may lead to the ignition coil overheating, causing it to malfunction or stop working properly.

4) Overheating

An overheated ignition coil may compromise its ability to conduct electricity effectively. This heat may be generated by different reasons, including prolonged engine operation under heavy loads, poor airflow around the ignition coil, or engine overheating.

5) Damaged Spark Plug

faulty Spark Plug

When the spark plug of your car becomes worn, it necessitates the ignition coil to work at a significantly higher output which may lead to its failure. Maintaining the spark plugs in good condition can help mitigate the risk of ignition coil failure.

When the spark plugs degrade, the gap each spark plug fires across expands, requiring the coil to generate a higher voltage to link this gap. This extra load on the ignition coils could lead to voltage overload, causing overheating and, ultimately failure.

How to Test an Ignition Coil?

The most straightforward method to test the ignition coils involves swapping one with another from a different cylinder to check if the issue follows the coil.

For instance, if a trouble code signifies a misfire on cylinder #1 and you interchange the ignition coil of cylinders #1 and #2, a faulty coil should then generate a trouble code for cylinder #2.

Follow the below-given steps to test the ignition coil:

  • Ensure your car is parked on a flat surface.
  • Allow the engine to cool off completely.
  • Disconnect the ignition coil from the car’s electrical system, which can be accomplished by removing the battery’s negative terminal.
  • Visually inspect the ignition coils for any noticeable damage, such as a leak or a crack. If any damage is found, it’s advisable to replace the ignition coil.
  • With the help of a multimeter, check the resistance of the primary winding by determining the resistance across the negative and positive terminals of the ignition coil. Based on the ignition coil type, the resistance should range from 0.4 to 2 ohms.
  • Check the resistance of the secondary winding using a multimeter and measure the resistance between the positive terminal and the high-tension output terminal. Based on the ignition coil type, the resistance should be from 6,000 to 15,000 ohms.
  • Check for voltage output. Attach a spark tester to the ignition coil’s high-tension output terminal. Next, turn on the engine and monitor the spark tester. If the spark appears strong and steady, it indicates your ignition coil is in good working order. On the contrary, if sparks are weak, it means your coil is defective.
  • Test the resistance of the spark plug wires by using an ohmmeter. The resistance should be between 5,000 to 10,000 ohms per foot. If the resistance is more than this range, it means the spark plug wires are not within the proper limits, and you need to replace them.
  • After making all the replacements, perform a test drive to check the performance of the new ignition coils.

Ignition Coil Replacement Cost

The replacement cost of the ignition coils depends on the vehicle model, labor cost, and the part’s brand. The average replacement cost of the ignition coil is from $180 to $320. In this cost, the coil itself costs from $140 to $220, while the rest pays for the labor.

Ignition Coil Location

Ignition coils are typically situated on the topmost part of the engine, specifically on the cam/rocker cover. To reach the ignition coils, start by removing the decorative plastic engine cover.

Ignition Coil Location

In older vehicles, you can find the ignition coil positioned between the battery and a distributor.

FAQ Section

Can I replace an ignition coil myself?

The complexity of replacing an ignition coil largely varies according to your engine and mechanical skills. In some cars, replacing an ignition coil is a straightforward task. However, in most models, the replacement of the coils might be more challenging due to the location of the cylinders. If you’re uncertain, it’s advisable to contact a mechanic to handle this job.

Can you drive with a bad ignition coil?

Yes, you can drive with a faulty ignition coil temporarily; it is not recommended to drive for a long time. Driving with a bad coil may reduce the car’s performance, and it may cease to run at any moment. Additionally, driving under such conditions could severely damage your engine.

Can oil damage the ignition coil?

Yes, the oil may damage the ignition coil.

How often should ignition coils be replaced?

Typically, ignition coils can last between 80,000 to 100,000 miles in most car models. However, it’s important to note that ignition coils are not part of routine maintenance. They only need to be replaced when they begin to cause problems. It’s worth mentioning that certain factors, like a lean air-fuel mixture, can accelerate the wear of the spark plug, leading to earlier failure of the ignition coils.

Can I replace just one ignition coil, or should I replace all of them?

Certain manufacturers offer ignition coils in sets to facilitate easier replacement, although replacing them all together isn’t mandatory. Fixing the faulty coil will turn off the check engine light. But since there’s a likelihood of having to replace the other coils in the near future, it might be more convenient to replace them all simultaneously.

Can I reuse the ignition coil?

As for vape coils, yes, they can be cleaned and reused. However, it’s not always the suggested practice.

Can a spark plug damage an ignition coil?

Yes, worn, misfiring, or dirty spark plugs may lead to ignition coil failure.

Can a bad ignition coil cause a misfire?

Yes, a bad ignition coil is one of the major causes of the misfire.

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