How to Replace a Bad Spark Plug

Replacing spark plugs can involve removing multiple components, like the intake manifold and spark plug wire — which can be difficult in some vehicles.

Moreover, a bad spark plug replacement job can lead to engine damage, requiring expensive auto repair. That’s why it’s best to get a professional spark plug service.

Still, if you wish to take the DIY route, here’s a guide on how mechanics perform spark plug replacement: 

Firstly, a mechanic will need:

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

Note: Ideally, replace all the spark plugs together — even if all haven’t failed.

Here’s the process they’ll follow:

1. Ensure the engine is cool. Then disconnect the battery.

2. Remove the engine cover.

3. Disconnect the ignition coil connector or the spark plug wire of the spark plug they’ll replace first. They’ll ensure not to tear off the wire terminal from the plug wire end. If the wire is damaged, they’ll replace it.

4. Unscrew the old spark plug using a spark plug socket.

5. Clean the spark plug hole to remove debris. This debris can damage the cylinder head threads.

6. “Gap” the new spark plug using a spark plug gauge if it isn’t “pre-gapped”. They’ll check your owner’s manual for the engine’s plug-gap specifications.

7. Apply a thin layer of dielectric grease to the spark plug boot.

8. Install the new spark plug.

9. Tighten the new plug with a torque wrench according to the manufacturer’s specifications.

10. Reinstall the ignition coil or spark plug wire connector.

11. They’ll repeat the process for the other plugs.

12. 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?

Getting spark plugs replaced with new inventory can cost between $100 and $500 (including car parts and labor costs). The cost of installing new plugs can depend on your location, vehicle model, and type of spark plug.

Here are individual estimates for new plugs (based on type):

• Copper spark plugs: $2 to $10 per piece

• Platinum spark plugs: $10 per piece

• Double platinum spark plugs: $20 per piece 

• Iridium spark plugs: $20 to $100 per piece

• Spark plug wires: $140 to $175

Note: You may also need to replace spark plug wires if you drive an older vehicle with a distributor ignition system.

How Often Will I Need to Replace Spark Plugs?

Most manufacturers recommend getting spark plug service or new inventory every 30,000 miles. However, your spark plug’s lifespan can range between 10,000 miles and 150,000 miles depending on the:

• Spark plug metal type
• Spark plug design
• Vehicle’s make and model

Consult your owner’s manual to know when to change spark plugs and the correct type for your vehicle. It’s also good to check the ignition coil and spark plug wires when replacing your old spark plugs. 

What if they go bad?
Get faulty spark plugs replaced immediately since 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 get your spark plugs checked while doing car maintenance and tune-up activities for other car parts, like brake pads or oil change service.

What Are the Common Symptoms of Failing Spark Plugs?

Faulty spark plugs can lead to automotive symptoms like:

1. Check Engine Light Comes On

A worn spark 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 spark plug wire or coil pack issues.

2. Fuel Efficiency Drops

Bad spark plugs increase your vehicle’s fuel consumption due to incomplete combustion.

In fact, spark plug failure can reduce your car’s fuel efficiency (a.k.a. fuel economy) by around 30%.

Unburned fuel from incomplete combustion that’s expelled from the combustion chamber may also damage the catalytic converter.

3. Engine Idles Rough

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, likely stalling it. However, the starting troubles could also be due to a car battery problem.

5. Engine Performance Reduces

An old spark plug can cause incomplete combustion, reducing engine performance and causing poor acceleration.

6. Exhaust Emissions Smell like Gasoline

Unburned fuel reaches the vehicle’s exhaust system when a spark plug issue doesn’t ignite the air-fuel mixture properly in the combustion chamber. This makes emissions smell like gasoline.

2 Spark Plug Replacement FAQs

Here are 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 spark plug gap grows so much that the required spark no longer forms, and the spark plug fails.

2. Do I Need to Replace All the Spark Plugs?

Ideally, you should change all the spark plugs together.

If you only replace bad spark plugs, each spark plug in your engine will have a different replacement cycle. Consequently, you’ll need to spend more effort on car care and auto repair whenever each spark plug fails.

Moreover, your car engine works best when all the spark plugs have similar performance levels. For instance, if two spark plugs in a four-cylinder engine perform differently than the rest, it can lead to the engine running out of sync.