How to Replace a Bad Spark Plug: Step-by-Step

Spark plugs are small engine components that ignite the air-fuel mixture to power your vehicle.

Replacing them requires having the right tools and enough access to remove surrounding components — whether that’s the spark plug wire, or in some engine designs, even the intake manifold. Because the job can be challenging on some vehicles, a poorly performed spark plug replacement job can lead to engine damage, necessitating expensive auto repairs. 

That’s why it’s best to get a professional spark plug service.

But, if you’re still curious about the process, here’s a look at how mechanics perform spark plug replacement:

Firstly, a mechanic will gather:

• New spark plugs
• Spark plug socket
• Ratchet and torque wrench
• Spark plug gauge (gap gauge)
• Anti-seize lube
• Dielectric grease for spark plug boot

Once they have the right tools, they’ll:

1. Ensure the engine is cool, then disconnect the battery and remove the engine cover.
2. Disconnect the ignition coil connector or remove the spark plug wire for the plug being replaced, ensuring that they don’t pull or tear the wire terminal. If the wire or connector is damaged, they’ll replace it too.
3. Unscrew the old spark plug using a spark plug socket, taking care not to damage the spark plug threads in the cylinder head.
4. Clean the spark plug hole to remove debris. Leftover debris can damage the spark plug threads or the cylinder head threads during installation.
5. “Gap” the new spark plug using a spark plug gauge if it isn’t “pre-gapped”. Refer to the owner’s manual for the engine’s plug-gap specifications.
6. Apply a thin layer of dielectric grease to the spark plug boot.
7. Install the new spark plug.
8. Tighten the new plug with a torque wrench according to the manufacturer’s specifications.
9. Reinstall the ignition coil or spark plug wire connector.
10. Repeat the process for the other plugs.
11. Reinstall the engine cover and start the engine to ensure everything works after installing the new plug set.

How Much Does Spark Plug Replacement Cost?

Modern spark plugs can cost between $100 and $400 (including parts and labor costs).

The cost of installing new spark plugs can depend on your location, vehicle model, and type of plug. Here are individual estimates for new plugs (based on type):

• Copper spark plugs: $5 per plug
• Platinum spark plugs: $10 – $15 per plug
• Double platinum spark plugs: $15 – $30 per plug
• Iridium spark plugs: $20 – $100 per plug

NOTE: You may also need to replace spark plug wires if you drive an older vehicle with a distributor ignition system. Spark plug wires can cost an extra $220 to $605.

7 Common Symptoms of Failing Spark Plugs

Faulty spark plugs can present the following warning signs:

1. Illuminated Check Engine Light

A worn or old plug can cause your Check Engine Light to activate or keep flashing.

Why?
Fouled spark plugs won’t ignite the air-fuel mixture correctly, triggering engine misfires and activating the Check Engine Light.

Besides a faulty spark plug, the Check Engine Light can also appear due to issues with the spark plug wire or coil pack.

2. Drop in Fuel Efficiency 

Bad spark plugs increase your vehicle’s fuel consumption as they cause incomplete combustion. In fact, spark plug failure can reduce your car’s fuel efficiency by around 30%. 

3. Rough Idle

If your vehicle has a faulty spark plug, you may face rough idling and hear pinging, rattling, or knocking sounds due to misfires.

4. Car Struggles to Start

Worn spark plugs will struggle to fire your engine, potentially causing it to stall. However, the starting troubles could also stem from a problem with the car battery, so it’s best to have a mechanic diagnose and repair the issue.

5. Reduced Engine Performance

Because an old plug can cause incomplete combustion, you may experience reduced engine performance and poor acceleration.

6. Exhaust Emissions Smell like Gasoline

When spark plugs fail to ignite the air-fuel mixture properly, unburned fuel enters the exhaust system, causing the emissions to have a gasoline-like odor.

This unburned fuel that escapes the combustion chamber can also damage the catalytic converter.

7. Physical Damage (Requires Inspection)

Any visible wear or damage on a spark plug means it’s due for a repair or replacement. Look for:

• Cracked insulators: Chips or cracks in the ceramic casing weaken spark delivery.

• Worn electrodes: Eroded or melted electrodes can’t ignite the air-fuel mixture effectively.

• Carbon or oil buildup: Heavy carbon deposits hinder spark, while oil deposits often indicate internal engine problems.

Is it Safe to Drive with a Bad Spark Plug?

No. It’s recommended to replace faulty spark plugs immediately, as they can cause poor acceleration and engine misfires. 

The latter could damage the catalytic converter, decrease fuel economy, and cause irreversible engine damage. Your car battery may also die from repeatedly trying to crank your car.

That’s why you should always have your spark plugs checked during general car maintenance, whether it’s an oil change, brake pad service, or even a fuel pump replacement.

How Often Do You Need to Replace Spark Plugs?

Most modern spark plugs require replacement every 30,000 to 100,000 miles. Check your vehicle’s manual for the recommended interval and the correct plug type. For example, copper spark plugs typically need replacement every 20,000 miles, while iridium plugs can last up to 100,000 miles.

However, spark plugs can fail earlier than their expected lifespan due to:

• High temperatures: Continuous exposure to intense heat can slowly erode the electrodes or, in severe cases, cause them to melt.

• Repeated high-voltage firing: Each spark requires a strong electrical charge, and over thousands of cycles, this stress wears down the plug’s internals.

• Combustion deposits: Residue from the combustion process can accumulate on the electrode, insulator, or inside the spark plug hole, limiting the plug’s ability to fire cleanly.

• Engine vibration: Ongoing vibration can weaken or crack the insulator, gradually reducing the plug’s durability and performance.

TIP: It’s also a good idea to inspect the spark plug wire and ignition coil pack when replacing your old spark plugs.

3 FAQs on Spark Plug Replacement

Here are the answers to questions that may arise when facing a spark plug issue:

1. Why Do Old Spark Plugs Need Replacing?

Every time a spark forms between the spark plug’s center and ground electrodes, it burns off metal from each electrode, creating a gap. 

This spark plug gap increases as your plugs fire during engine operation. Over time, the gap grows so much that the required spark no longer forms, and the spark plug fails.

2. What Causes Spark Plugs to Go Bad?

Spark plugs can fail due to underlying mechanical issues or simply from normal wear over time. 

Common causes include:

• Oil leaking into the combustion chamber
• Overheating from pre-ignition or a cooling system malfunction
• Carbon fouling (black, dry soot) caused by a dirty air filter or a rich air-fuel mixture
• An incorrect spark plug gap

3. Do You Need to Replace All the Spark Plugs?

Yes, it’s recommended to replace all spark plugs simultaneously. Replacing only the bad ones creates different wear cycles, meaning you’ll end up doing more frequent repairs as each plug fails at a different time. 

Your engine also performs best when all spark plugs fire with the same efficiency. If even one or two plugs in a four-cylinder engine are weaker than the rest, the engine can run unevenly or out of sync, affecting smooth combustion and overall ignition timing consistency.