Symptoms of a Bad Ignition Coil & How to Fix It


Ignition coils are a vital part of your car’s ignition system, and if they happen to stop working, you’ll definitely notice it. Because your car’s ignition coils help your engine properly combust the air/fuel mixture in the cylinders, bad ignition coils will cause your engine to run abnormally.

Luckily, it’s pretty easy to diagnose a bad ignition coil, and it’s also a pretty easy issue to fix. In this article, we’ll go over everything you should know about your ignition coils, including how they work, how to tell if you have a bad one, and how to fix them if they stop working.

What Is an Ignition Coil?

An ignition coil is a component of the ignition system in gasoline engines. Diesel engines don’t use ignition coils, since they use compression alone to combust their air/fuel mixture.

The ignition coils in your car are essentially power transformers that take the low voltage from your car’s battery and turn it into a high voltage current for the spark plugs. This is necessary for the spark plugs to actually generate a spark hot enough to ignite the fuel in the cylinders.

It’s a little tricky to explain how an ignition coil works in simple terms, but it basically works like this: in an ignition coil, you have two sets of wire coils, which are the primary and secondary coils. Both sets of coils loop around a core, but the secondary coil contains many more loops than the primary coil.

When a current from the battery is passed through the primary coil, it generates a magnetic field. Interrupting the current causes the magnetic field to collapse, which causes the current to flow into the secondary coil. Because the secondary coil contains more loops than the primary coil, this multiplies the original voltage of the current. 

Some engines from back in the day used only a single ignition coil hooked up to a distributor for all of their spark plugs, but these days it’s far more common to have individual ignition coils for each spark plug.

An engine needs both working ignition coils and spark plugs in order to function, and bad ignition coils and spark plugs can cause a lot of the same symptoms.

What Makes an Ignition Coil Fail?

The most common reason ignition coils fail is because of bad wires or burnt-out spark plugs. Your coils might also fail prematurely if your air/fuel ratio is off, or if they’ve experienced excessive vibrations or heat.

In general, however, ignition coils are pretty durable, and only need to be replaced every 80,000 to 100,000 miles.

Bad Ignition Coil Symptoms

Below are all the possible symptoms of a bad ignition coil. It’s worth noting that a lot of these symptoms are also symptoms of bad spark plugs, so it’s probably worth it to check both your ignition coils and your spark plugs if you notice these symptoms occurring. 

Related: Can You Drive a Car With a Bad Fuel Injector?

Since the ignition coil gets its power from the battery, a malfunctioning battery can also cause some of these symptoms. In any case, if these symptoms start happening, you can be fairly sure that the problem lies somewhere in your car’s ignition system.

1. Exhaust Backfiring

Most of the symptoms caused by a bad ignition coil have to do with the fact that fuel is not being properly combusted within the cylinders, and exhaust backfires are no exception. Backfiring happens when unburnt fuel manages to pass through the cylinder and combusts in the exhaust pipe.

You’ll know your car is backfiring if you hear loud bangs or pops coming from the exhaust, or if you notice that your exhaust is black and smoky or has a strong smell of fuel. Prolonged backfiring can seriously damage your car’s exhaust system, so you should have this issue fixed as soon as you can unless you’re cool with paying a hefty repair bill. 

A backfiring engine will also produce more emissions than normal, so a bad ignition coil can cause you to fail a smog check as well.

2. Trouble Starting the Engine

If one or more of your ignition coils have stopped working, you’ll have a much harder time starting your engine, particularly if you have an older car with a single ignition coil and a distributor.

Related: 5 Reasons Why A Car Engine Knocks On Startup Then Goes Away

Without working ignition coils, there will be no spark in the cylinder to ignite the fuel, so when trying to start the car the engine will crank but nothing will happen. 

3. Poor Fuel Economy

If you notice that your fuel economy is a lot worse than it should be, this may also be the result of a bad ignition coil. A faulty ignition coil can negatively affect your fuel economy in a couple of ways. 

Like we mentioned earlier, your fuel economy may be going down because fuel is making its way through the engine without being used. Your engine’s computer may also be partially at fault here; if it’s sensing that your spark plugs aren’t operating at full capacity, it may try to make up for it by sending more fuel to the cylinders.

4. Engine Misfires or Idles Roughly

An engine that misfires, idles roughly, or otherwise runs abnormally in any way may be the victim of bad ignition coils. Engines need a steady supply of fuel and consistent combustion in order to run correctly, and if it’s missing one or both of these things, you’ll definitely notice it.

An engine misfire feels kind of like the engine is hiccupping a little bit; the engine will jerk without warning, and it’ll feel like you’ve lost power for a brief instant. You may also hear unusual popping sounds coming from the engine.

5. Engine Fails to Make Power

If you step on the gas and nothing seems to be happening, a bad ignition coil could be the reason why. As you surely know by now, ignition coils are an essential component of your engine’s combustion process, and if combustion isn’t happening correctly, the engine will fail to produce its usual power.

6. Check Engine Light is On

If one or more of your ignition coils have failed, odds are that the check engine light will come on to let you know. However, the check engine light can turn on for a wide range of issues, and not necessarily for issues that relate to the engine itself. 

If you really want to get down to the bottom of why your check engine light is on, you’ll need an OBD-II scanner. An OBD-II scanner plugs into your car’s computer and can display for you your computer’s error codes.

To find out for sure whether your problems are being caused by a bad ignition coil, you’re going to want to be on the lookout for a specific range of error codes. If it is indeed your ignition coils that are causing the problems, you should keep your eyes open for codes P0350 to P0362.

How to Fix a Bad Ignition Coil

If you’re not confident enough to work on your car yourself, there’s no shame in taking your car to a repair shop to fix your ignition coils. If you do plan on fixing your ignition coils yourself, however, it’s generally an easy and uncomplicated process.

We should mention that it’s not actually possible to fix a busted ignition coil; if your coils have indeed failed, they’ll need to be replaced.

Fortunately, you don’t need much equipment to replace your ignition coils. All you need is a basic toolkit, a multimeter, and of course new ignition coils.

Here’s the step-by-step process of how to replace your ignition coils:

  1. With the engine cool, open the hood and disconnect the negative battery cable from the battery.
  2. Locate the old ignition coils. In many cases, the coils will be accessible without disassembling the engine further, but in some cases, you may have to remove the intake manifold to get at the coils.
  3. Once you’ve located the ignition coils, start by disconnecting the plastic electrical connector from the coils, then unscrew the bolts that hold the coils in place. 
  4. With the bolts removed, you can pull the coils out of the engine block. Be careful when removing the ignition coils from the engine, as you can damage the spark plugs if you’re too rough. 
  5. To test your coils, use a multimeter to measure the resistance of each coil. If you find that the coil has way more or less resistance than normal, it’s a sign that the coil has gone bad. Your owner’s manual should tell you what the correct resistance for your coils should be.
  6. Make sure you have the right replacement coils for your car. If you do, you can install them now. Start by applying some dielectric grease to the inside of the ignition coil insulator boot, and then carefully push the new coil back onto the spark plug.
  7. Reinstall the ignition coil bolts and reconnect the electrical connectors. Reassemble the engine manifold (if you took it apart) and reconnect the negative battery cable to the battery.
  8. Take your car out for a test drive. If everything runs fine, you’re good to go. If not, it’s possible that your problem lies somewhere other than your ignition coils.

How Much Does It Cost to Fix an Ignition Coil?

This depends on the car you’re buying a new ignition coil for. Low-end coils may cost as little as $75, while high-end coils may be as much as $300.

You also have to factor labor costs into account, so in general, you should expect to pay at least $150 or so to get your coils replaced at a repair shop.

Types of Ignition Coil Systems

Over the decades, there have been a few different types of ignition coil systems that have existed. Let’s briefly touch on each of them and see how they differ.

Conventional Systems

Conventional ignition systems are the oldest and most basic type of ignition coil systems. In a conventional ignition system, power flows from a single ignition coil to a mechanical distributor, which directs the power to the spark plugs.

While conventional ignition systems are simple to make and cheap to fix, they also wear out very quickly and have thus fallen out of favor among car manufacturers.

Electronic Systems

Electronic ignition systems are essentially the same as conventional ignition systems, with one major difference. Instead of using a mechanical distributor to send power to the spark plugs, these systems use an electric distributor that uses a pickup coil to regulate current delivery instead of a distributor cam and points. 

Electronic ignition systems are more reliable than conventional systems, but not as much as distributorless systems.

Distributorless Systems

As the name implies, distributorless systems don’t use a distributor at all; instead, the ignition coils sit directly on top of the spark plugs and are actuated by the car’s computer. Distributorless systems may use either one coil per spark plug or one coil per two spark plugs.

Distributorless ignition systems are the newest type of ignition system, and also the most reliable. This is due to the fact that distributorless ignition systems contain no moving parts, which means they wear out much more slowly.

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