You turn the key. The engine cranks, but it feels rough. Maybe the whole car is shaking at a red light, or you’re putting your foot down to pass someone, and the power just isn't there.

It’s frustrating, but it’s usually not a dead engine. Nine times out of ten, it’s an electrical issue—specifically, the coil ignition system.

At PartsBlue, we see this all the time. A customer thinks their transmission is slipping because the car is jerking, but it’s actually just a $50 part that decided to quit. This guide will walk you through exactly how coil ignition systems work, how to troubleshoot an ignition coil in your driveway, and whether you can get away with a cheap replacement.

How the Coil Ignition System Actually Works

Think of your car battery like a garden hose. It has flow (12 volts), but it doesn't have the pressure to jump a gap.

The ignition coil is the nozzle that creates the pressure. It takes that 12-volt power from the battery and ramps it up to 20,000 or even 50,000 volts. It holds that charge for a split second and then—snap—sends it down to the spark plug ignition coil connection.

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That high-voltage pulse jumps the gap at the tip of the spark plug, creating the explosion that pushes the piston down. If the coil is weak, the spark is weak. If the spark is weak, the fuel doesn't burn, and your car runs like a tractor.

Types of Ignition Coil Systems

Not every car uses the same ignition coil setup, and knowing which type you have can make diagnostics faster.

Single Coil + Distributor

Older cars used one ignition coil feeding a distributor, which then sent a spark to each cylinder. If the coil failed, the entire engine would die. These systems are simple but far less efficient.

Coil Pack Systems

Common in late-90s and early-2000s vehicles, coil packs fire two cylinders at once (wasted spark system). When one coil in the pack fails, you’ll usually lose two cylinders instead of one.

Coil-On-Plug (COP) Systems

Most modern engines use one coil per spark plug. This setup gives better spark control and efficiency, but it also means each coil lives directly on top of the engine—right where heat and vibration are highest. That’s why individual coil failures are so common today.

Bad Ignition Coil Symptoms:

You usually don't need a mechanic to tell you a coil is bad. The car screams it at you. Here are the most common symptoms of a failing ignition coil:

• The "Misfire" Jerk: This is the big one. It feels like the engine hiccups or stumbles while you're driving. It’s most obvious when you are under load (like driving up a hill).
• Rough Idle: You're sitting at a stop sign, and the steering wheel is vibrating. Can a bad ignition coil cause a rough idle? 100%. It’s basically an engine running on one less cylinder than it should.
• Gas Smell: If the spark doesn't fire, raw fuel gets dumped into the exhaust. You might smell gas or see black smoke.
• The Check Engine Light: If this light is flashing, pull over. A flashing light means the misfire is bad enough to melt your catalytic converter.

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How Do You Check an Ignition Coil?

You don't need expensive tools for this. If you are wondering how to test an ignition coil, just use the "Swap Method." It’s free and foolproof.

• Scan it: Use a cheap OBD2 scanner to find the code. If it says "P0302," that means Cylinder 2 is misfiring.
• Mark it: Pop the hood. Find the coil on cylinder 2.
• Swap it: Take the coil from cylinder 2 and swap it with the coil from cylinder
• Clear and Drive: Clear the code and drive the car until the light comes back on.
• Check again: Scan the codes. Did the error move to "P0301" (Cylinder 1)? If yes, the coil is dead. If the error stayed at Cylinder 2, your coil is fine, and you likely have a bad spark plug or fuel injector.

Using a Multimeter

If you want to get technical on how to check the ignition coil with a multimeter:

Set your meter to Ohms. You’re checking resistance.

• Primary (Input pins): Should be low (around 1.0 Ohm).
• Secondary (Output tip): Should be high (6,000+ Ohms). If you get a reading of "Infinity" or zero on the secondary side, the internal winding is snapped.

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What Causes Ignition Coils to Fail?

Ignition coils don’t just die randomly. There’s usually a reason.

Heat Soak
Coils sit in one of the hottest places in the engine bay. Over time, heat breaks down the internal insulation and windings.

Worn Spark Plugs
As spark plugs wear, the gap gets wider. That forces the ignition coil to generate a higher voltage to jump the gap. Eventually, the coil overheats and fails.

Oil or Water Intrusion
Valve cover gasket leaks can fill the spark plug well with oil. Moisture or oil around the coil boot causes misfires and internal arcing.

Cheap Electrical Components
Low-quality coils often use inferior windings and insulation. They may work fine at first, then fail suddenly under load.

Replacement Costs and Tips

So you found the bad apple. Now you need to handle the ignition coil replacement.

Can I replace ignition coils myself?

Absolutely. If you can unscrew a bolt, you can do this.

On most engines, the coil sits right on top. You unplug the connector, remove the 10mm bolt, pull the old one out (it pops like a cork), and slide the new one in. How long does it take to replace one ignition coil? Maybe 10 minutes if you take your time.

Ignition Coil Replacement Cost

If you go to a dealer, they might charge you $200 per coil plus labor. Don't do that.

• Price for ignition coil replacement (DIY): A solid aftermarket coil costs between $30 and $60.
• Labor: Shops charge high rates for diagnostics. Doing the "Swap Test" yourself saves you that diagnostic fee immediately.

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Quick FAQs

How much does it cost to replace a coil ignition?

DIY cost is roughly $40-$80, depending on the car. Shop cost is closer to $200-$300.

Do I need to replace ignition coils with spark plugs?

It’s smart to do them together. Old spark plugs have a wider gap, which forces the coil to work harder to jump it. That extra strain is usually what kills the coil in the first place.

Can I drive with a bad ignition coil?

Technically, yes, but you risk destroying your catalytic converter. It’s better to park it or limp it home gently.

Does an ignition coil affect acceleration?

Yes. If a cylinder isn't firing, you are losing 25% of your power (on a 4-cylinder engine). The car will feel sluggish and heavy.

What is the life expectancy of an ignition coil?

They usually last about 100,000 miles, but heat and vibration can kill them sooner.

Got a misfire you can't pin down? Check out the ignition coils at PartsBlue for high-output coils that keep the spark alive.