How to Detect a Bad Ignition Coil & When to Replace It

Technical

Jan 24th, 2022

How to Detect a Bad Ignition Coil & When to Replace It

Most car owners are quick to suspect the engine when the car won’t start, but did you know that it could also be a bad ignition coil?

What Is an Ignition Coil & What Does It Do?

An ignition coil, also sometimes called spark coil, is a high-voltage low-current transformer that plays a crucial role in engine management. 

It converts voltage from the 12V car battery into 25,000V to 30,000V to create a spark in the spark plug. This spark ignites the fuel-air mixture in the engine, starting the vehicle. If the ignition coil is faulty, the car won’t start. 

There is no single way to determine that your ignition coil is faulty, but it doesn’t mean you can’t tell when you have a bad ignition coil. Several symptoms can help you detect a faulty ignition coil, and in this article, we look at the common ones.

7 Symptoms that Help You Detect a Bad Ignition Coil

1. Check engine light comes on

The engine light on the dash usually turns on when there is an issue with the engine. Since the ignition coil’s function correlates with the engine’s, illumination of the check engine light could imply there is a problem with the coil. 

The engine code that typically shows up on the diagnostic tool when the ignition coil has a problem is P0351 (Ignition Coil – Primary/Secondary Circuit Malfunction). 

2. Engine backfires

Engine backfires usually point to early stages of ignition coil failure. Backfiring happens when unused fuel leaves the internal combustion cylinders through the exhaust pipe. It is also associated with a bad gasoline smell and black smoke from the exhaust pipe. If left unchecked, backfiring can lead to more serious and costly damages, especially through the exhaust system

3. Engine stalling

A problematic ignition coil can result in engine stalling due to irregular sparks that can’t ignite the fuel-air mixture. If you notice engine stalls after driving for a few miles, chances are your ignition coil is faulty. If not fixed, engine stalls could make your car stop and shut off completely. 

4. Poor fuel economy

Poor fuel economy is another common sign of a bad ignition coil. Having a faulty ignition coil means more power needs to be channelled to the spark plug, which in turn leads to high fuel consumption by the engine. This translates to mileage drop, so if you notice your car’s mileage going down, consider checking the ignition coil.

5. Loud engine noise

It is usual for the engine to make some noise, but also listen for bad ignition coil sound. Loud engine noise accompanied by vibrations could indicate a faulty ignition coil that doesn’t generate enough voltage, thus overworking the engine.

6. The car does not start at all

Isn't it too obvious at this point that your ignition coil could be having a problem? A faulty ignition coil can altogether prevent the car from starting since the engine won’t be turned on. 

You can tell if the problem is with the engine coil or not when you start the car. If there’s a clicking sound when you start the vehicle, the ignition has no issue. But if there is no sound at all, there’s a likelihood of failure of the ignition system. 

7. Engine misfires

A misfire occurs when one or more combustion cylinders fail to combust the fuel-air mixture, possibly be triggered by a bad ignition coil or spark plug. Often, misfires result in audible noises like coughing or sputtering noise.

Misfires can be caused by several reasons such as spark delivery problem, timing issues, or even fuel delivery problems. To be safe, check the ignition coil when you notice an engine misfiring.

If you suspect an issue with the ignition coil of your car, you can carry out an ignition coil test for confirmation purposes. Note that the test depends on the coil you use, i.e., CNP (Coil-Near-Plug) or COP (Coil-On-Plug). 

Replacing the Ignition Coil in 5 Easy Steps

If the ignition coil test confirms that the ignition coil is faulty, the thing to do would be to replace it. 

With the right tool, you can carry out the ignition coil replacement procedure from your garage at home. You’ll need a pair of rubber gloves and an appropriate wrench/spanner. Here are the steps to follow:

1. Turn off the car engine

If you just got home from driving, you must turn off the car engine first and let the car cool down. Ignore this step if your vehicle hasn't been on the road. 

2. Disconnect the battery

Remove the negative terminal of the battery to disconnect it. 

3. Remove the ignition coil

Locate the ignition coil(s) usually at the top of the engine and remove it. After you've removed it, disconnect the electrical connectors from the coil.

Warning: In some cars, you might need to disconnect the electrical connectors before removing the ignition coil.

4. Connect the new ignition coil

Set the new ignition coil in place and tightly fit all the screws/bolts you might have unfastened. Reconnect the electrical connectors you had removed earlier.

Similarly, if you had to disconnect the connectors before the coil, reconnect them first and then set the new ignition coil in place.

5. Reconnect the battery

The last step is to reconnect the negative terminal back to its position and close the hood.

Once you’re done with everything, turn the car engine on and take the car for a quick spin to ensure you carried out the replacement correctly. If you did it correctly, there would be no idling, backfires, or stalling.

Know When to Seek the Pros

We recommend going to a professional auto mechanic if you're uncomfortable doing the replacement yourself. Find mechanics and repair shops the easy way by using our automotive-dedicated directory here. Choose the one nearest you for convenience.

The average ignition coil replacement cost is $200-$350, whereas the ignition coil itself will cost you anywhere between $80-$150 depending on its type and your car model.

Replacing car parts and accessories comes with your responsibilities as a car owner. Know where to go and whom to call even before emergencies occur. So, it helps if you have a mechanic that you could call or go to during these times. 

You should also know how to find replacement parts quickly. CarPart AU provides a quick way for car owners to connect to the most reliable auto part suppliers in Australia. This service is free, and all you need to do is describe the part in detail in a request form, and we'll find the best sellers for you. Use CarPartAU's Request-a-Part tool now


By Sam O.