Ignition coils are vital for starting your car’s engine. Simply put, coils amplify battery voltage, so the spark plugs have enough power to ignite the air/fuel mixture inside the combustion chambers.
Older cars use a device known as a distributor to “distribute” power to each spark plug in sequence. Modern vehicles instead use coil packs, which are essentially a collection of ignition coils.
A bad ignition pack will not only reduce performance but can also harm other engine parts. For these reasons and more, you should address a faulty coil pack at the first sign of an issue.
What are the symptoms of a bad coil pack? The seven most common include:
- Rough idle
- Reduced performance
- Check engine light
- Engine backfires
- Poor fuel economy
- Motor struggles starting
- Engine stalling
This guide covers these symptoms in more detail. Moreover, we’ll also review coil pack replacement costs, which can run up to $300 in some models. Finally, we’ll explain how to test a coil pack at home and replace it yourself.
Note: Faulty coil packs are not the only cause of the symptoms mentioned in this article. Before replacing any engine parts, be sure to confirm the culprit. Additionally, the coil pack replacement costs described are estimates. The exact pricing depends on the make and model of your vehicle.
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7 Bad Coil Pack Symptoms
Symptom 1: Rough Idle
One of the more noticeable bad coil pack symptoms is a rough idle. Usually, a rough idle stems from an issue in the ignition system, including but not exclusive to a faulty coil pack.
A car that’s idling rough will appear to have trouble staying running. Telltale signs of a rough idle include inconsistent RPMs, shaking, and misfiring. The motor may also make slipping sounds as it struggles to produce power.
Symptom 2: Reduced Performance
A bad coil pack can also cause sluggish performance, including hesitation while accelerating. For your car’s engine to produce power, it must create a high-voltage spark to ignite the air/fuel mixture inside the combustion chambers.
Moreover, each spark plug must fire in the proper sequence to maintain combustion. A faulty coil pack can affect this ignition timing, as well as combustion power, leading to reduced performance.
Symptom 3: Check Engine Light
As with many car problems, a check engine light is another “symptom” that may appear if a coil pack goes out. Why? Because a faulty coil pack won’t be able to generate enough voltage to fully ignite the air/fuel mixture in the combustion chambers.
If the coil pack is the culprit, a multimeter tool will likely display a P0300, P0302, or P0351 error code. The first two refer to engine misfiring problems, and the third to a coil pack malfunction.
Symptom 4: Engine Backfires
Have you ever heard an older vehicle make a loud “bang” sound? That’s what backfiring is, and it occurs when the air and fuel inside the combustion chambers ignite outside the cylinder walls.
There are several reasons a car might backfire, including the engine running rich, incorrect engine timing, and—you guessed it—a faulty coil pack. Regardless of the cause, backfiring should be addressed promptly to avoid harming other engine components.
Symptom 5: Poor Fuel Economy
A car’s fuel economy says a lot about its overall condition. So, if you start noticing a reduction in gas mileage, it may be due to a problem with one of the coil packs.
If a coil pack fails, the O2 sensor may get an incorrect reading. This can cause the system to try and compensate by burning more gas, leading to a drop in miles per gallon.
Symptom 5: Motor Struggles Starting
Your car’s battery doesn’t provide enough power by itself to start the engine. This is where coil packs come in, amplifying the low-voltage current so it’s sufficient. For this reason, if a coil pack fails, don’t be surprised if the motor struggles starting.
A few symptoms you may experience when starting a vehicle with a bad coil pack include jerking, sputtering, and vibrating. The engine can still start, but you may need to hold the key longer than usual.
Symptom 7: Engine Stalling
Another bad coil pack symptom to watch for is engine stalling, which is not only inconvenient but can be hazardous as well. For these reasons, if your engine begins stalling, you should address it before causing an accident.
Like other symptoms on this list, a faulty coil pack can cause an engine to stall because it compromises the combustion process. The engine may stall if the coil packs don’t provide the spark plugs with adequate voltage.
Coil Pack Replacement Costs
Can you repair a faulty ignition coil in a pack? Yes, but economically it makes more sense to simply replace the bad coil.
A shop will take 30 minutes to an hour to replace an ignition coil at an average rate of $75 to $130 per hour. As for parts, an individual ignition coil should run between $40 to $125. This comes to about $77.50 to $255 to replace an ignition coil before taxes and fees.
The automotive research site RepairPal shows a similar cost range of $211 to $286.
To save on costs, you can also replace a coil pack yourself. You’ll only pay for parts in this case, so $40 to $125. For the best experience buying car parts online, consider sticking with reputable stores like Advance Auto Parts, Rock Auto, and Auto Zone.
How To Test A Coil Pack
A multimeter is a handheld diagnostic tool for reading manufacturer error codes. You can test your car’s coil packs yourself if you have one.
Each ignition coil has a pair of wire windings, one the primary and the other the secondary. The primary should measure between 0.4 and 2 Ohms and the secondary 6k to 20k Ohms. Be sure to check your vehicle’s owner’s manual for exact specifications.
To read the primary winding, you’ll need to set the Kiloohms scale to 200. Next, touch one probe to the ignition coil’s positive post and one to the negative. Polarity does not matter, as you’re just reading the resistance.
For the secondary winding, set the multimeter’s Kiloohms scale to 20k. Then, touch one probe to the center post and the other to either the positive or negative. Again, polarity does not matter for testing.
If readings are within range, your car’s ignition coils are fine. However, if the figures are outside, the coil has likely failed and should be replaced.
For a visual guide to testing an ignition coil, check out the below video from YouTube’s SnapOnMark:
How To Replace A Coil Pack Yourself
Coil pack replacement is simple. With your engine turned off and the faulty coil located, you can start by disconnecting the terminals and labeling them if necessary.
To remove the coil, you should be able to simply pull it out, though some models require undoing a bolt first.
Place the old coil aside, insert the new one, attach the terminals to the posts, and you’re done.
Auto Repair Guys provide a solid visual guide in their YouTube video below:
Keep Coil Pack Repair Costs To A Minimum By Addressing Symptoms Early
If your car experiences any of the symptoms mentioned in this guide, your best bet is to address them promptly.
Ignoring bad ignition coil signs can cause long-term damage to other parts. This will likely result in repair costs far exceeding that of a faulty coil.
Thankfully, ignition coils tend to last about 100,000 miles before having issues. To ensure your ignition coils last as long as possible, be sure to follow a routine maintenance schedule.