Written By: Don Dodi
Fact Checked By: Kristen Brown
Reviewed By: Diego Rosenberg
Actually spark plug is the one which sparks the fire: as whenever you turn the key then the spark plug that ignites a mixture of air and fuel in the engine’s cylinders that then creates the combustion and that’s how it powers your vehicle.
And, yes these are the tiny but really powerful components that really affects the car’s engine performance directly.
Also, the significant thing about it as a single spark plug can actually fire hundreds of times per minutes: which is over 3 million sparks per hour of driving.
And that’s why over the time, these sparks actually wear down the electrode and that leads to the poor fuel efficiency, engine misfires, rough idling and even you will face trouble to start your car.
Not only this, according to the U.S. Department of Energy, a worn-out spark plugs can actually reduces the fuel efficiency by as much as 30% which actually means that you could be wasting money every time you drive with them.
Additionally, most of the manufacturers actually recommends the spark plugs to be replaced in every 30,000 to 100,000 miles, yes it does depends on the type as well: copper, platinum or iridium.
And to replace them the method is really simple and can be done easily and you’ll know the really effective method on this guide which is all about How To Replace Spark Plugs.
So, when you replace them it will help keep your engine healthy and also reduce your fuel costs.
Symptoms of Worn or Failing Spark Plugs
Worn or bad spark plugs can cause a variety of engine problems that affect how your car drives, the fuel it uses and its reliability.
Recognizing these symptoms early can help you avoid bigger and costlier problems later. Here are a description of the most common warning signs:
1. Engine Misfires
An engine misfire occurs when one or more of the engine’s cylinders don’t work properly. This causes the engine to shake, stumble, or jerk—specially when you’re accelerating or going uphill.
Engine misfires can cause damage over time and this is often caused by dirty, worn spark plugs or a failure to spark at the right time.
2. Poor Acceleration
If your car feels sluggish or takes longer than normal to pick up speed, it could be because the spark plugs aren’t producing a strong enough spark to ignite the air-fuel mixture.
This causes the engine to respond slowly and weakly, specially when you press the gas pedal sharply.
3. Decreased Fuel Economy
Bad spark plugs don’t burn fuel efficiently. When combustion is incomplete, your car has to work harder and burn more fuel to do the same amount of work. This leads to lower miles per gallon (MPG) and higher fuel costs over time.
4. Rough Idling
If your car shakes or vibrates when it’s standing still (like at a red light), this is called rough idling. It often feels like the engine is stalling or not running smoothly. This is a common sign that one or more spark plugs are worn out or bad.
5. Difficulty Starting the Engine
Spark plugs play a direct role in starting your car. When they become worn or damaged (clogged with carbon or oil), the engine may take longer to crank, or the engine may not start at all. This problem is specially common in cold weather.
6. Illuminated Check Engine Light
When your car’s computer detects a problem with the engine — such as a misfire caused by a bad spark plug — it can turn on the check engine light on your dashboard.
This light can mean a number of things, but the spark plug is one of the most common culprits, specially if other symptoms are present.
Tools
| Tool / Material | What It Is | Why You Need It / What It Does |
|---|---|---|
| Socket Wrench | A hand tool used for turning bolts and nuts. | Helps you loosen and tighten spark plugs during removal and installation. |
| Spark Plug Socket | A special socket containing rubber. | Designed to hold and protect the spark plug while removing or installing it. |
| Spark Plug Gap Gauge | A small device that measures the space (gap) between the electrodes on a plug. | This ensures that the spark plug gap is correct for your vehicle, which is important for proper engine ignition. |
| Torque Wrench (Recommended) | A wrench that clicks when a specific tightness is reached. | Prevents overtightening or undertightening of spark plugs, which can cause damage or loose plugs. |
| Anti-Seize Compound (Optional) | A silver-like coating applied to threads. | Helps prevent spark plugs from getting stuck in the engine, but only use it if the spark plug manufacturer recommends it. |
| Dielectric Grease | A clear, waterproof grease used on electrical parts. | Protects the spark plug boot and wires from moisture and improves electrical contact. |
| New Spark Plugs | Fresh spark plugs suitable for your engine. | Replacing old plugs restores engine performance. Make sure they are the correct type and size for your car. |
| Compressed Air (Optional) | A can or air tool filled with air that is used to expel debris. | It helps to clean out dirt and dust around the spark plug holes before removing the old plugs, preventing debris from falling inside. |
| Service Manual / Spec Sheet | A book or PDF that lists your vehicle’s specifications. | Tells you the correct spark plug type, gap size and torque settings for your specific car model. |
Preparation Before Replacing Spark Plugs
Before you begin replacing your spark plugs, it is very important to take the correct safety and setup steps.
These steps protect both you and your vehicle and also ensure that the process goes smoothly without damaging the engine parts. These are the important things you should keep in mind before touching any spark plug.
1. Park on a Flat, Cool Surface
Always park your car on a level surface like a driveway or garage. Make sure the engine is completely cool before you start it.
Working on a hot engine can burn your hands and cause spark plugs to become stuck due to the expansion of the metal.
2. Disconnect the Negative Battery Terminal
To avoid electrical shock or short circuit, disconnect the negative (-) terminal of the battery before starting work.
This step ensures that no electricity flows through the engine system, specially when you are handling the spark plug wires or ignition coils.
3. Locate the Spark Plugs (Varies by Engine Layout)
Depending on your car’s engine type (e.g., 4-cylinder, V6, or V8), the location of the spark plugs may vary.
In 4-cylinder engines, the plugs are usually lined up on the top or side of the engine. In V6 or V8 engines, they can be difficult to reach and are located on either side of the engine block.
4. Clean Area Around Spark Plug to Prevent Debris from Falling In
Before removing any spark plug, use a clean, dry cloth or compressed air to clean the area around the plug hole.
This helps prevent dirt, dust or small debris from falling into the engine cylinder when removing the spark plug, which could cause serious engine damage.
How To Replace Spark Plugs – Step-by-Step Instructions
Replacing spark plugs is a simple process if you follow the right steps in the right order.
Patience and care are a must, as working with engine parts requires attention to detail and precision. Here’s a step-by-step guide to walk you through the entire process:
1. Remove Ignition Coils or Spark Plug Wires
Before you can access the spark plugs, you’ll need to remove the ignition coils or wires connected to them.
If your car uses separate ignition coils (which is common in cars today), unplug the electrical connector and remove the coil. For cars with spark plug wires, remove them by gently pulling them out of the spark plug boot.
Label each wire or coil with masking tape or a marker so you can connect them to the correct cylinder later. Changing the order could cause engine trouble or damage.
2. Remove Old Spark Plugs
Use a spark plug socket attached to a socket wrench and ratchet extension to remove the old spark plugs. Insert the socket deep into the plug well and turn anti-clockwise to loosen.
Be careful when loosening. Spark plugs are often made of brittle ceramic and can shatter if force is applied. If a spark plug breaks, the remaining parts can be difficult and costly to remove.
3. Check and Adjust the Gap (if Necessary)
Not all new spark plugs come pre-gapped. Use a spark plug gap gauge to measure the distance between the center and ground electrodes on the end of the plug.
This gap controls the size and strength of the spark. If it’s too wide or too narrow, it can cause a misfire, weak spark, or poor engine performance.
4. Install New Spark Plugs
Before inserting new spark plugs, make sure they are clean and undamaged. Begin by threading each plug into its hole by hand to avoid cross-threading, as this can cause permanent damage to the engine head.
This is actually optional, as some people apply a small amount of anti-seize compound to the threads, but only if the spark plug manufacturer recommends it. Many modern spark plugs already have the special coating and do not need anti-seize.
Once the plugs are hand-tightened, use a torque wrench to tighten them to the manufacturer’s recommended specifications.
Over tightening can strip threads or crack the ceramic; under tightening can cause engine leaks or loose spark plugs.
5. Reattach Ignition Wires or Coils
Once all the spark plugs are installed, it is time to reconnect the ignition wires or coils.
If you’re working with spark plug wires, apply a small amount of dielectric grease to the inside of the rubber boot to protect it from moisture and ensure a good electrical connection.
Then press the wire or coil boot firmly onto each spark plug until it clicks or fits tightly.
You should always double-check that each wire or coil is connected to the correct spark plug. Mixing them up can cause misfiring and poor engine operation.
Post-Installation Check of Spark Plug
Once you’ve installed new spark plugs and reconnected all wires or ignition coils, it’s important to double-check everything before driving the vehicle.
These steps help ensure that the spark plugs are working properly and that your engine is running smoothly. Skipping this step can cause problems such as misfires or reduced fuel efficiency. Here’s what you should do after installation:
1. Reconnect the Battery
First, reconnect the battery’s negative terminal, which you disconnected earlier. This will return power to the car’s electrical system.
Make sure the terminals are properly tightened. A loose battery connection can cause starting problems or display an error message on the dashboard.
2. Start the Engine and Listen for Misfires or Rough Idling
Turn the key or press the start button to start the engine. Pay close attention to the sound and feel of the engine.
- Does it start easily?
- Is the engine running smoothly?
- Do you hear any popping, knocking or vibration noises?
If the engine feels dry, makes a knocking noise, or makes unusual noises, it means one of the spark plugs is not seated correctly, is not gapped correctly, or is not connected to the correct coil/wire.
As well as, when you adjust the spark plug gap that really influences the engine behaviour, as a wider gap literally produces a stronger spark which basically improves the power and that lowers the fuel usage as well. But however, when gap is too wide then it can actually lead to the misfires and unwanted increases in NOₓ emissions as well and engine noise as well, specially at the high rpm: which is why you should tune the gap according to the manufacturer’s specifications which is really essential to work it in the perfect way.[¹]
3. Check Dashboard Lights
After the engine starts, check any warning lights on the dashboard, specially the check engine light (CEL).
- If the CEL comes on, use an OBD-II scanner to read the error code.
- Codes like P0300-P0308 usually indicate a misfire in one or more cylinders, which could be caused by incorrect installation.
If you don’t have a scan tool, many auto parts stores offer free code scans to help identify the problem.
4. Test Drive and Monitor Performance
Take the car for a short, gentle test drive—preferably on a quiet street or parking lot.
And just note:
- Smooth acceleration
- Responsive throttle
- No engine hesitation or jerking
- Steady idling at stops
- Improved fuel efficiency over time
Driving for 10-15 minutes gives the engine control unit (ECU) a chance to “relearn” the new spark plug firing behaviour, specially in modern vehicles with adaptive ignition timing.
Read More:
How To Maintain Spark Plug Health and Engine Performance
Proper spark plug maintenance not only helps your engine run smoothly—it also extends the life of your engine, improves fuel efficiency and saves money in the long run.
Adopting these simple and smart habits can help you avoid unexpected problems and keep your car in top condition. Here are some important maintenance tips every driver should follow:
1. Use OEM or High-Quality Spark Plugs
Always select OEM (original equipment manufacturer) spark plugs or a trusted high-quality aftermarket brand.
These are specifically made to suit your car’s engine design and combustion requirements. Using cheap or the wrong plugs can result in poor performance, early wear, misfires or even engine damage.
Check your owner’s manual or service manual for the exact part number and specifications of the spark plugs.
2. Replace All Plugs at the Same Time
If only one spark plug is bad, it is best to replace them all at once. This keeps the engine balanced and ensures even combustion in all cylinders. Mixing old and new plugs can cause uneven engine performance and put stress on other parts.
In fact, uneven spark strength from old vs. new plugs can cause rough idle, poor fuel economy and long-term wear on the ignition coil.
3. Keep a Maintenance Log
Maintaining a simple service log helps you keep track of when the spark plugs were last changed and what brand or part number you used.
This is specially useful if you own your vehicle for a long time or if you plan to sell it and want proper maintenance.
Note the mileage, date of replacement, plug type and gap setting so you can easily refer to it the next time you replace it.
4. Inspect Ignition Coils and Wires During Plug Changes
When changing spark plugs, take a moment to inspect the ignition coil or spark plug wire. Look for signs of cracks, corrosion, burning, or wear. Bad coils or wires can cause a weak spark or misfire—even if the spark plugs are new.
In fact, bad ignition coils are one of the leading causes of misfires and can also damage new spark plugs if they aren’t replaced when needed.
So, if your car has more than 100,000 miles on it, it’s a good idea to consider replacing old coils or wires as a preventive measure.
Conclusion – How To Replace Spark Plugs
When you successfully change the spark plug of your vehicle, then you’ll also feel that it has a big impact on how your engine runs.
Now, this guide has actually showed you everything from spotting early signs of spark plug trouble, to preparing your workspace earlier, from choosing the right tools, to installing the new plugs with perfection.
And when you maintain the spark plug and check them at the regular time interval then it will protects your engine and also by doing this simple routing checkup you can save lot of money in the long run as well.
Frequently Asked Questions
Q1. What are signs of bad spark plugs?
Answer: Bad spark plugs can cause a number of serious problems in your car. One of the most common signs is engine misfire, which means your engine may shake or stall while you’re driving. You may also find your car has a hard time accelerating, specially when you press the gas pedal. Another telltale sign is poor fuel economy—if you’re visiting the gas station more often than usual, worn spark plugs could be the cause. Uneven or erratic driving is also a common symptom of a car stopped at a red light. Sometimes, your car may have trouble starting or may take longer to crank than usual. In some cases, the check engine light comes on, often because the car’s computer detects misfiring or incomplete combustion. All of these are signs that the spark plugs are not working properly, affecting engine performance.
Q2. Can you drive a car with bad spark plugs?
Answer: Yes, you can drive a car with bad spark plugs, but it is not recommended. Although the car may still run, driving with worn or damaged spark plugs can lead to more serious engine problems over time. The engine may not operate properly or have reduced power and may have frequent misfiring. When spark plugs don’t fire correctly, your engine has to work harder, which puts more strain on other parts like the ignition coils, fuel injectors and even the catalytic converter. This can lead to costly repairs. Bad spark plugs also reduce fuel efficiency, which means you’ll have to spend more money on petrol. If the misfiring is severe, your car may stall or not start at all. So, even if the car is technically drivable, continuing to drive with bad spark plugs can shorten your car’s life and cause major damage in the long run. It’s best to replace them as soon as you notice any problems.
Q3. What is the lifespan of spark plugs?
Answer: Spark plug life depends on the type of spark plug and the vehicle manufacturer’s recommendation. Generally, standard copper spark plugs last about 20,000 to 30,000 miles. Platinum spark plugs typically last about 60,000 miles, while iridium spark plugs, which are made of harder and more heat-resistant material, can last 100,000 miles or more. However, these numbers can vary depending on your driving habits and the condition of the engine. For example, frequent short trips, poor fuel quality, or engine trouble can cause spark plugs to wear out sooner than expected. It’s always a good idea to check your vehicle’s owner’s manual to learn the recommended replacement interval. Regular maintenance checks can also help you spot early signs of wear, so you can replace the plugs before they cause major problems.
References:
[1] The Effect of Variations in Spark Plug Electrode Tips on Power and BMEP of Motorcycle Engines
https://www.internationaljournalssrg.org/IJME/2023/Volume10-Issue7/IJME-V10I7P102.pdf
Guys, I’m a car audio enthusiast and customization expert and I love clear sound and clean installation. Plus, I have spent years helping people build their dream audio setups. And whenever I’m working on cars, I probably listen to music and thinks about the next big upgrade.