How to Spot a Bad Ignition Coil

Mia Bevacqua
April 11, 2018


A faulty ignition coil can cause several problems for your engine:

1. Check engine light comes on: The car’s computer oversees coil pack operation. If it detects a problem with an ignition coil, it will turn on the check engine light and log any related trouble codes.

2. Your engine runs poorly or stalls: A coil pack that’s not firing properly will result in incomplete engine combustion. This causes the engine to misfire, resulting in a vehicle that hesitates, shudders and bucks. The car may even stall, especially at idle.

3. Increased emissions: Increased hydrocarbon emissions can result from a bad coil pack. This can cause a vehicle to fail a state emissions inspection.

4. Reduced fuel economy: Incomplete combustion usually causes poor fuel economy. A bad coil pack may result in more frequent trips to the gas station.

5. Car won’t start: On vehicles that have just one ignition coil for all the spark plugs, a coil problem can keep the car from starting at all.

Get it diagnosed by a professional

What does the ignition coil do?

The ignition coil, also known as the secondary coil, is a transformer that converts primary battery voltage (low voltage) into the secondary voltage (high voltage). This high voltage fires your car’s spark plugs, which help create combustion in your engine. Most cars made in the past three decades have one ignition coil, or coil pack, per cylinder. Others have just one coil for all the cylinders.

Inside the ignition coil are two sets of windings. The primary coil windings contain hundreds of turns of heavy wire, while the secondary side contains thousands of turns of fine wire. 

Battery power flows through the primary windings. When your car’s computer decides it’s time to fire that cylinder’s spark plug (or plugs), it cuts the current to the primary windings, causing the energy in them to collapse into the secondary windings. This multiplies the voltage, which is then sent to the spark plug in order to make it fire.

As technology has improved our cars, the ignition system has changed dramatically. Each new type of ignition system has brought the ignition coil closer to the spark plug. A coil now sits directly on top of each spark plug for each cylinder.  

This works much better — here’s an example: Let’s say a V8 engine fires four of its spark plugs every time the crankshaft makes one full turn. If just one ignition coil is used for all eight cylinders, and the engine is turning 6,000 RPM, that coil must fire one plug every 2.5 milliseconds — that’s 400 sparks per second. By having eight ignition coils to split up the load instead, the job becomes much easier.

» MORE: Get an estimate for your ignition coil replacement

How to fix the problem

If your car is showing these symptoms, you should get it diagnosed by a professional technician. There are several problems that can cause these issues, and it’s not always easy to tell what’s at fault without some investigation.  

Your mechanic will use a diagnostic scan tool to check any trouble codes logged by the car’s computer. These will help indicate whether the ignition coil is the likely culprit, or whether another part may be failing.

With ignition coils that fire at very high speeds, the only way to accurately test a coil’s output is with an oscilloscope. Professional technicians now have handheld scopes that are often integrated into the high-end scan tools used to fix modern cars. 

If one or more coil packs are faulty, they should be replaced. Continuing to drive with bad ignition coils can damage other parts, including the catalytic converter.

In most vehicles, the coils are pretty easily accessible, and replacing the coil should only take minutes. In some cases, especially for cars with individual coil packs installed on each cylinder, removing the rear coil packs may be problematic. In these scenarios, the engine mounts may even have to be removed to create access, and this job would be best left to a professional mechanic.

Since a faulty ignition coil causes a misfire, it will also cause the related spark plugs to foul. Always replace the spark plug or plugs that the bad coil was firing. This will restore the power and fuel economy.

Some manufacturers recommend replacement of the ignition coils at scheduled intervals. Check your owner’s manual to see if this is the case for your car.


Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

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