Symptoms of a Bad Ignition Coil: All that You need to know

Ignition coils are the metal parts responsible for the electronic management of a vehicle’s ignition. Therefore, malfunctioning ignition coils can lead to critical mechanical and performance problems in your car. The symptoms of a bad ignition coil are mostly manifested as difficulties starting the car, a stalling engine, and loss of power while on the road.

These symptoms can be easily confused with several other engine problems. Therefore, we need to know how to diagnose a bad ignition coil.

Initially, when your car engine misfired and your “check engine” light turns on, you knew you had a problem with your distributor. Nowadays, most modern cars replaced the distributor with a coil pack, thus, increasing electronic transmission efficiency. However, because a coil pack ignition system involves fewer moving parts, identifying ignition coil problems and their correction poses challenges to a car owner.

overheating ignition coil symptoms

What Does an Ignition Coil Do?

A vehicle ignition coil is a transformer that amplifies the small voltage from a car battery into enough power capable of igniting the fuel and starting the engine.

In detail, ignition coils are electronic devices that comprise of two wire-coils wrapped around a metal core. Since a car ignition coil acts as a step-up transformer, the secondary coil has many more turns than the primary coil.

When an electric current flows from the battery to the primary coil, it gets repeatedly disrupted by the secondary coil creating a strong magnetic field that charges the secondary coil to a higher voltage than the initial 12 Volts that came from the car battery.

The coils then fire the sparkplugs directly upon command from the engine computer, unlike the conventional system that had to use a distributor for power transmission.

Nowadays, there are car sensors and ignition modules that inform the engine control unit of the car when to open or close the coil current. Modern cars also have timing lights to monitor the sequence of sparkplugs.

Symptoms of a Bad Ignition Coil

Symptoms of a Bad Ignition Coil

As described above, ignition coils handle one of the essential processes of a car. Therefore, motorists should look out for ignition coil failure causes and the symptoms of coil malfunction for early correction. Here are some of the signs that your ignition coil is about to fail.

1. Hard Starts

The ignition system needs to spark at the right time for the engine to ignite and the vehicle to run. Therefore when your engine experiences hard starts, it means that the ignition coils are faulty and are not delivering the required voltage to the sparkplugs. You can also experience hard starts when you have bad sparkplugs. Hard starts are more common with single-ignition coil engines.

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2. Check Engine Light is Turned On

Vehicles will illuminate the check engine light immediately there is a misfire or a hard start during ignition. Most drivers ask “will a bad coil pack show a code?” The answer is yes. Modern vehicles with OBD-diagnostic scanners will display the P0351 engine code, which means there is a malfunction with the ignition coil primary and secondary circuits. Other cars may show different codes, so you are better placed consulting with your repair shop when you get the code.

3. Misfiring Engine

When the ignition coil fails, the car starts but won’t stay running. It produces sputtering and coughs like sounds when you try to start it. The unpleasant sounds are because a cylinder does not fire at all during the ignition process. It also results in violent vibrations and jerking motions. The misfiring behavior of a failed ignition coil resembles the effects of failed sparkplugs or problems with the fuel delivery system.

An engine can misfire at any point when you have a bad ignition coil. Although, more often, misfires happen when you accelerate because you subject the engine to more load.

Misfires generate emissions that are damaging to the catalytic converter and other parts of the engine. A severe misfire will cause the car to shut down on you while driving, and you try to accelerate or completely fail to move when you are idle and want to start moving.

Transmission problems and clogged EGR valves can also cause misfiring.

4. Poor Gas Mileage Control

There are times when the ignition fails spontaneously, and there are times when it fails gradually. A gradually failing ignition coil seems to perform normally but has a detrimental effect on your fuel economy. You can notice that by checking the amount of mileage, you get whenever you fuel your car. If the mileage is progressively reducing, you may have a bad ignition coil.

The mileage progressively reduces because the vehicle injects more fuel to compensate for the low energy that the ignition coils dissipate. The mileage also decreases when you have a bad ignition coil because the oxygen sensor fails to get correct readings leading to more fuel injection.

5. Backfires

Backfires are sudden loud bangs caused by incomplete combustion and the release of fuel through the exhaust system. When bad ignition coils are unable to produce enough voltage for the spark plugs to transmit to the fuel, the fuel explodes loudly.

These bad ignition coil sounds mostly burst through the exhaust, but when the explosions occur inside the vehicle, they can cause extreme damage and injury.

Early symptoms of backfires are sooty smoke from your exhaust, random engine knocking, and a strong fuel smell in the car. Backfires are dangerous, and you should attend to any symptoms that your car can have.

6. Diminishing Power Output

Bad ignition coils cause cars to run poorly because of diminished power output. The car breathes roughly hen idle and accelerates sluggishly. These early symptoms should remind you to fix your car to avoid further damages and more expenses.

This diminished power output is due to irregular sparks to efficiently bring about the required fuel combustion to operate the vehicle. The result of low power output is fewer rotations per minute visible through the sluggish acceleration and poor engine performance of the car.

7. Oil Leaks

Oil leaks are one of the hot ignition coil problems. Faulty ignition coils overheat because they are unable to transmit the energy from the battery to the sparkplugs efficiently. Overheating breaks engine pipes, causing oil leakages.

How to Test Ignition Coil

Ignition coil testing is dangerous and requires a lot of care. Follow these steps carefully when you think you have a problem with your ignition coil.

  • If your vehicle was running, ensure you give it enough time to cool before opening the hood. It would be best if you had your tools box for the steps that follow.
  • After opening the hood, remove the negative terminal on the battery to avoid electrocution. Make use of your underhood work light to identify the terminal if you have poor visibility.
  • Spot the ignition coils on the engine and remove the bolts holding them.
  • Confirm the recommended primary and secondary winding voltages from your car’s user manual.
  • Now test the thicker primary winding first by connecting the positive and negative terminals of your multimeter to the corresponding terminals on the winding’s coil.
  • Check if the readings on the multimeter tally with the recommended readings in your manual.
  • A higher than-recommended reading indicates that the coil was open while a zero reading suggests that the coil short-circuit.
  • Now test the secondary winding of your ignition circuit similarly by connecting corresponding terminals to the multimeter. Confirm the readings you get with the ones on your manual to find out if it malfunctioned.
  • If your car has multiple ignition coils, testing all of them is the only way how to tell which ignition coil is bad.
    Most times, ignition coils malfunction because of spark plugs or they may return good readings but have physical damage and weakened insulation. Therefore, instead of rushing to replace the coils, test the other parts to know what causes ignition coils to keep going bad

Find out the recommended sparkplug resistance ratings from your user manual and test them using an ohmmeter to determine whether their values fall within the compliance figures. Also, ensure that the sparkplug wires are correct and in order.

How to Change Ignition Coils

Running with a bad ignition coil is damaging to your car. Follow the following steps to replace your malfunctioning coil with a new one.

1. Take safety precautions

Make sure your car has cooled down before opening the hood. Assuming that you have read and understood your vehicle’s user manual, go ahead and disconnect the negative terminal of the battery before you touch anything else. Please note that some cars recommend that you use a memory retainer if you intend to keep your car dismantled for a long time.

2. Remove the coils

The most difficult part about removing ignition coils is you have to go through obstacles like the sparkplug and coil insulator boots to get to them. If yours is the kind of car that you have to remove the plenum manifold to access the ignition coils, then you need to have a new gasket. Carefully remove the ignition coil bolts without damaging the electrical plastic connector. Twist gently to slide and remove the insulator boot from the sparkplug.

Perform all these actions with care because if the insulator boot tears and a piece remain at the sparkplug, it will cause a misfire the next time you start the car. Remove one coil at a time and identify each with a picture, number mark, or coil pigtail so that you don’t confuse them during the assessment.

Thoroughly check for signs of oil or any contamination in each coil insulator boot and fix the leaks before replacing the ignition coils.

3. Test Each Ignition Coil

You need to test the ignition coils because sometimes when the check engine light flashes, it only reports a misfire. Something else apart from the ignition coils might have caused the misfire. Look for a code reader or find an auto shop to find what the code symbolized.

The code will relate the misfire to the cylinder with the problem, and from there, you can identify the ignition-related part that caused the misfire. If you removed multiple coils and you identified the ignition coil side with the misfire, try switching the coils to see if the malfunction follows the suspected coil to the other side. If it does, then you can proceed to step number 4. That is also one way how to tell if the ignition coil is bad.

If the misfire doesn’t follow the coil, test both coils using a multimeter as explained earlier to identify the faults.

4. Install the New Coils

Lubricate your new ignition coils before you install them. Confirm that you have the right oil. Also, double-check the terminal pins and pigtail connections. Apply dielectric grease to the insulator boot of the new ignition coil to provide a barrier against moisture. The grease will also aid to reduce friction the next time you need to remove the coil.

Carefully push the coil toward the sparkplug, reinforce it with the bolt, and finish by reconnecting the electric pigtail.
You can now return the components you removed and test if your ignition coil replacement is successful by taking your car for a test drive.

Symptoms and causes of a bad ignition coil YouTube Video

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Q: How long do ignition coils last?

Ideally, ignition coils should last around 100,000 miles. However, several factors can reduce that mileage. Ignition coils work symbiotically with other engine parts like sparkplugs. When one part malfunctions, it overworks the other. Therefore, the lifespan of ignition coils is determined by the overall condition of the engine.

Q: Can an ignition coil get weak?

Yes. Ignition coils can get weak from vibrations due to the vehicle’s motion or heat conditions. Heat and vibrations open the primary and secondary windings of ignition coils, reducing their efficiency.

Another critical thing that can weaken ignition coils is the high resistance of the spark plugs. Sparkplug resistance causes the ignition coils to get very hot and makes them burn through the insulation causing electrical shorts.

Q: How much does it cost to replace ignition coils?

The cost of replacing an ignition coil depends on the car model you have. You can find high-end vehicles with ignition coils of up to $350 and low-end vehicle ignition coils of around $70.

Replacement costs also depend on whether you want to do it yourself or you want a professional to do it for you. Professional labor costs are around $150 to $200.

Q: Should you replace ignition coils with sparkplugs?

Most of the time sparkplugs and ignition coils get damaged together or are the reason for the damage. Always test the sparkplugs whenever you are replacing ignition coils. Although it isn’t always necessary to replace sparkplugs together with ignition coils, it is pointless to install new ignition coils and leave the sparkplugs that caused the coils to malfunction.

Q: What color is a weak spark?

A yellow or red spark is a weak spark that symbolizes low voltage unable to spark the cylinder to ignite the vehicle. A blue spark is a strong spark with enough energy to ignite the car.

Final Words

Ignition coils are essential electromagnetic devices that convert the 12V from a car battery to around 100000V. 100000V is enough to power the sparking of the cylinders in the engine to ignite and facilitate fuel combustion for the car to operate. Any problems with the ignition coils can invoke severe issues with vehicle operations.

That is why car owners need to monitor any symptoms of a bad ignition coil. These symptoms include engine misfires, hard starts, poor mileage control, and backfires.

Luckily, the replacement procedure for ignition coils is simple, as outlined earlier. Don’t wait for your car to develop worse problems. Fix it as soon as you spot the symptoms outlined.


Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

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