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


May 22nd, 2020

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

The ignition coil is a crucial electronic engine management component that serves an instrumental role in any car. It is a high-voltage low-current transformer that converts voltage from the 12V car’s battery into 25,000V to 30,000V to create a spark in the spark plug. This spark ignites the fuel-air mixtures in the engine, starting the vehicle. If the ignition coil is faulty, the car won’t be able to start. 

Most car owners usually suspect the engine when the car won’t start without giving some thought to the ignition coil. Unfortunately, there is no specific way to know that your ignition coil is faulty. That, however, 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.

How to 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 has 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. However, 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. Often, misfires lead 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 engine misfire.

If you suspect an issue with the ignition coil of your car, you can carry out a couple of tests 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

Once you’ve confirmed that the ignition coil is faulty, the next step is replacing it. With the right tool, you can carry out the 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. If you’re just from driving first, turn off the car engine and let the car cool down for some time. Ignore this step if your vehicle hasn't been on the road. 

2. Remove the negative terminal of the battery to disconnect it. 

3. 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.

Note that in some car, you might need to disconnect the connectors before removing the ignition coil.

4. 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 on.

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

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

Replacing car parts and accessories is all part and parcel of owning a car. 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 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 CarPart's Request-a-Part tool now

By Sam O.