What you need to do before you start

An engine coil converts low voltage from your battery into the high voltage needed to fire your spark plugs. When one fails, your engine misfires, your check engine light comes on, and you lose power. Replacing it is a job most owners can do in 30 minutes to an hour, depending on how many coils your engine has and where they sit.

Before you touch anything, let the engine cool completely. A hot engine can burn your hands, and some coils sit near hot exhaust manifolds. Disconnect the negative terminal of your battery using a wrench — this prevents accidental shorts while you work. You do not need to remove the battery itself, just the cable.

Locate the coil or coils you need to replace. Most modern cars have one coil per spark plug, mounted directly on top of each plug (called coil-on-plug). Older cars may have one or two coils mounted on the engine block or firewall, with wires running to each plug. Your owner's manual or a repair manual for your specific year and model will show you exactly where yours sits.

Key Takeaways

  • Let your engine cool and disconnect the negative battery terminal before you start work.
  • Most modern cars have one coil per spark plug mounted directly on top of each plug; older cars may have one or two coils with wires running to each plug.
  • You will need a socket set, a wrench, and a new coil matched to your vehicle's year, make, and model.
  • The coil unplugs from an electrical connector and bolts or clips to the engine; reverse the order to install the new one.
  • If your check engine light returns after replacement, the original problem may have damaged the spark plug or ignition wire.

Tools and parts you will need

Gather a socket set with a ratchet, an adjustable wrench or the correct wrench size for your coil's bolt, and a new coil. The coil must match your vehicle's year, make, model, and engine size — buying the wrong one wastes money and time. Check your owner's manual or ask the parts counter at an auto supply store for the correct part number.

You may also want a coil puller tool if your coil is stuck tight, though most come free with the new coil or can be borrowed from an auto parts store. A flashlight or headlamp helps if you are working in low light. Have a clean rag nearby in case you need to wipe dirt or oil from the connector.

Removing the old coil

Locate the electrical connector on the coil — it is usually a plastic plug with a release tab or clip. Press or pull the tab to disconnect it. Some connectors are tight; if it will not budge, wiggle it gently side to side while pulling. Do not yank the wire itself, as that can tear the connector inside.

Once the connector is free, remove the bolt or bolts holding the coil to the engine. Most coils have one bolt on top; some have two. Use your socket set or wrench to turn the bolt counterclockwise. Keep the bolt in a safe place — you will use it again. Lift the coil straight up and out. If it is stuck, rock it gently back and forth; do not force it, as the rubber seal can tear.

Look at the spark plug underneath. If the coil was stuck because of corrosion or carbon buildup, wipe the top of the plug with a clean rag. If the plug looks damaged, black, or wet with oil, note that — you may need to replace it as well.

Installing the new coil

Before you install the new coil, check that the rubber seal on the bottom is clean and not cracked. If the old seal stuck to the plug, gently peel it off the plug and discard it. The new coil comes with a fresh seal.

Line up the new coil over the spark plug and push it straight down until it seats firmly. You should feel it click or snap into place. Install the bolt and tighten it snugly with your socket set or wrench — do not overtighten, as that can crack the coil housing. A snug fit is all you need.

Reconnect the electrical connector by pushing it onto the coil until you hear or feel it click. The connector should not wiggle. Reconnect the negative battery terminal by tightening the cable with your wrench.

What to do if the check engine light stays on

Start the engine and let it idle for a minute. If the check engine light goes out within a few seconds, the coil replacement worked. If it stays on or comes back on after a few minutes of driving, the problem may not have been the coil alone.

A bad coil can damage the spark plug or ignition wire underneath it. If the light returns, the spark plug may need replacement as well. You can check the plug yourself by removing the coil again and looking at it — a healthy plug has a light tan or gray tip, while a fouled plug looks black, wet, or heavily corroded. If you are unsure, a mechanic can read the error code and tell you what failed.

Coil-on-plug versus older coil systems

Most cars built after 2000 use coil-on-plug, where each coil sits directly on a spark plug. These are the easiest to replace because you only remove one coil at a time and do not have to deal with ignition wires.

Older cars may have one or two coils mounted on the engine block or firewall with thick wires running to each spark plug. If your car has this setup, the coil removal is the same, but you will also need to disconnect the ignition wires from the coil terminals. Mark each wire with tape before you remove it so you can reconnect them in the correct order — the order matters. Your repair manual will show you the correct firing order for your engine.

When to replace more than one coil

If your car has multiple coils and more than one is failing, you may see multiple misfire codes or a check engine light that does not clear after replacing one coil. This is common in older cars where all the coils are the same age and wear at the same rate.

If you replace one coil and the light returns within a few weeks or months, the others are likely failing too. Replacing all of them at once costs more upfront but saves you from repeating the job multiple times. Ask your mechanic or parts supplier whether your vehicle is known for coil failure — some models are more prone to it than others.

Frequently Asked Questions

Can I drive with a bad coil?

You can drive short distances, but you should not. A bad coil causes misfires, which waste fuel, reduce power, and can damage your catalytic converter over time. The longer you drive with a bad coil, the more expensive the repair becomes.

How much does a replacement coil cost?

A new coil typically costs between $25 and $150 depending on your vehicle. Labor at a shop adds another $50 to $200. Doing it yourself saves the labor cost and takes less than an hour.

What if I disconnect the battery and my radio or computer resets?

Yes, disconnecting the battery will erase your radio presets and may reset your car's computer. Write down your radio stations before you start, or reconnect the battery when ready after you finish the coil work. The reset itself does no harm.

Do I need to replace the spark plug when I replace the coil?

Not always, but it is worth checking. If the plug looks clean and in good condition, you can reuse it. If it looks black, wet, or corroded, replace it at the same time — the coil failure may have fouled the plug, and a fouled plug can cause the new coil to fail faster.

What does a coil puller tool do?

A coil puller is a metal bracket that hooks under the coil and uses leverage to pull it straight up without damaging the connector or the rubber seal. Most auto parts stores loan them free with a deposit, or they come included with some replacement coils. If your coil comes out easily by hand, you do not need one.