How to Change Spark Plugs
FixMyCar looks at the different types of spark plugs and how to replace them at home.
Book a car repair nowEllie Dyer-Brown
Thursday January 25 2024
6 min read
Spark plugs are vital in petrol engines, providing the spark needed to kick start the combustion process. They need replacing approximately every 30,000-90,000 miles, and with the right tools and knowledge, this is something you can do at home. Find out how in this guide.
What are spark plugs?
Spark plugs are a key part of a petrol car’s ignition system. They provide the spark that makes combustion possible, powering the engine. Diesel cars use glow plugs, which are slightly different.
Each spark plug has three parts: a metal threaded shell (often made of nickel or copper alloy), an earth electrode, and a centre electrode.
The process goes like this:
Fuel enters the cylinder via an intake valve.
The fuel is compressed by a piston and mixed with air.
Voltage is sent down the spark plugs, causing a spark to jump between the centre electrode and the earth electrode.
The spark ignites the explosive air/fuel mixture, which expands and pushes the cylinder down the piston.
You can learn more about how an engine works in this guide.
How to change spark plugs
Follow the steps below to fit your new spark plugs.
Equipment
New spark plugs compatible with your vehicle
Torque wrench
Spark plug socket spanner
An extender (if required)
Spark plug gap gauge (if needed)
Step one - Prepare
Find out where your spark plugs are located by consulting your vehicle handbook. They are usually either on top of or on the side of the engine. Clear any dirt from around them and make sure they are easily accessible.
Step two - Remove high-tension lead
Remove the first high-tension lead (also called an HT lead or ignition lead). These leads are the thick, insulated wires attached to the plugs. It's best to remove them one at a time as you change each spark plug to avoid mixing them up.
Step three - Remove old spark plug
This is where your spark plug socket spanner comes in handy. Use it to unscrew one of the old spark plugs (with the extension attached if needed).
Step four - Get new spark plug ready
Consult your vehicle handbook to find the correct electrode gap distance. Use the gap setting tool to ensure the distance is right, then put a smear of copper grease on the new plug threads and screw it in.
Step five - Tighten the plug
Use a torque wrench to tighten the plug. Your vehicle handbook will tell you what torque is required. Once it's in place, you can reconnect the high-tension lead.
Step six - Repeat
Repeat the above process for each spark plug. When finished, make sure you put back any parts you removed to access the plugs.
Changing spark plugs is something you can do at home, but if you don’t feel confident working under the bonnet, there are plenty of expert mechanics on FixMyCar who can help.
What spark plugs for my car?
There are a few types of spark plugs; choosing the right ones for your car is paramount. Many websites allow you to enter your registration plate and will then tell you the specific spark plugs you need. You can also find this information in your vehicle handbook.
Copper/nickel spark plugs
Many spark plugs contain copper or nickel in their centre because these are excellent metals for conducting electricity. However, the lifespan of these spark plugs is relatively short compared to others.
Single platinum spark plugs
Platinum-tipped spark plugs are slightly more pricey than some of their counterparts but have a much longer lifespan. Platinum doesn't erode in the same way that nickel does, so it is well-suited to modern car engines.
Double platinum spark plugs
These spark plugs aren't budget-friendly, but they offer enhanced performance - even more so than single platinum spark plugs.
Iridium spark plugs
Iridium is the most resistant to corrosion of all the metals listed above. It can withstand temperatures of up to 2,000 degrees. Spark plugs of this kind have a fine-wire centre that is better able to conduct electricity, and they can sometimes last as long as platinum spark plugs.
Silver spark plugs
Silver spark plugs create sparks of increased energy compared to many other types available on the market, and they also have the best thermal conductivity. Typically, they are fitted on older, European high-performance cars and occasionally motorcycles.
Five symptoms of bad spark plugs
Misfiring engine - this will cause your engine to lose power momentarily before regaining speed. You might also notice a loud popping sound and black smoke.
Trouble starting - without functioning spark plugs, your engine won't be able to start.
Slow acceleration - a common symptom when there's a fault with the ignition system.
Poor fuel economy - faulty spark plugs significantly impact fuel consumption.
Black exhaust smoke - unusual smoke usually accompanies engine misfires.
This guide discusses when the spark plugs in your car need replacing.
Spark plug tester
There are many different spark plug testers available to buy online. They are affordable and safe to use - here's how.
Remove the high tension (HT) lead and plug the device in-line. It should be connected to the end of the HT lead and the base of the spark plug.
Start the engine as you normally would. If the device lights up, this indicates a strong electrical input; if it doesn't, the spark plug likely needs replacing.
You can do this yourself or use FixMyCar to find a reliable local garage to do it for you.
Frequently asked questions
How long do spark plugs last?
Changing your spark plugs at the correct intervals is a good idea to ensure your engine can function as expected. Generally, they last around 20,000 to 30,000 miles.
During a full service, worn spark plugs will also be replaced. Find out the difference between a full service and an interim service here.
Do diesels have spark plugs?
Diesel engines work differently from petrol engines; for that reason, they have glow plugs rather than spark plugs.
Is there a spark plug removal tool?
Yes, you can purchase tools which are designed to make it easier to remove spark plugs. For example, spark plug spanners come in different sizes and are designed to fit into spark plug sockets.
What can I use as a spark plug cleaner?
If you're willing to spend a little extra cash, there are spark plug cleaning machines which plug into the air hose. However, this isn't necessary; instead, you can purchase cleaner spray for spark plugs for under £20.
What is an ignition spark plug?
An ignition spark plug is the same as a regular spark plug - sometimes, they are called plugs.
How does a spark plug work on an electric car?
Electric cars don't require spark plugs because they are battery-powered.
Not ready to change your own spark plugs? FixMyCar's garage network has plenty of mechanics who can do it for you.
If you found this guide helpful, you might also like:
- Related topics:
- Engine And Cooling
- How To
- Maintenance
- Spark Plug Replacement
Ellie is FixMyCar's Content Manager. She has over three years of experience writing about cars and regularly collaborates with automotive experts to provide trustworthy advice for drivers that is easy understand. Her work has been featured in Yahoo! Finance, iNews, The Daily Express and The Sun. She has a BA in English literature and an MA in creative writing from Durham University. Outside of work, Ellie follows F1 and eagerly awaits Ferrari's next era of dominance in the sport. She drives a Suzuki Swift.