What Is a Spark Plug? A Definitive Guide

If your engine runs, a spark plug is the reason it started. A spark plug is a small threaded device that delivers an electrical spark into your engine's combustion chamber, igniting the compressed air and fuel mixture that powers your pistons. Without a working set of spark plugs, a gasoline engine simply will not run. In this guide, we'll walk through exactly what a spark plug is, how it works, the warning signs of a failing one, how to test, clean, gap, and replace one yourself, and what it typically costs in Canada.

Table of Contents

1. What Is a Spark Plug and What Does It Do?
2. How Does a Spark Plug Work?
3. Do Diesel Engines Have Spark Plugs?
4. Types of Spark Plugs
5. How to Choose the Right Spark Plug
6. 5 Signs of a Bad Spark Plug
7. How to Test a Spark Plug
8. How to Clean a Spark Plug
9. How to Gap a Spark Plug
10. How Often Should Spark Plugs Be Changed?
11. How to Replace a Spark Plug (Step-by-Step)
12. How Much Does Spark Plug Replacement Cost?
13. FAQs

What Is a Spark Plug and What Does It Do?

A spark plug is an ignition system component that produces an electrical spark to ignite the air and fuel mixture inside your engine's cylinders, which is what actually starts and sustains combustion.

A spark plug screws into the cylinder head of your engine, with its tip protruding directly into the combustion chamber. Its fundamental job is to aid in the combustion process by fitting into the engine's cylinder head, where it's connected to the ignition coil. Every spark plug is built with a central electrode running through a ceramic insulator, and a grounded side electrode a small gap away.

Spark plugs are made from a mix of durable materials, since they need to survive thousands of tiny explosions every minute without breaking down. The metal shell is threaded so it screws directly into the cylinder head, and it's electrically isolated from the central electrode by a ceramic insulator, which is why you'll often see spark plugs described as having a "porcelain" look even though the working material is technically ceramic.

Without functioning spark plugs, your engine has no spark to ignite the fuel, and the engine will not run at all. That's why a full set of healthy spark plugs is considered one of the most critical, if inexpensive, parts in your entire vehicle.

How Does a Spark Plug Work?

A spark plug works by receiving high-voltage electricity from the ignition coil, then releasing that energy as a spark across a small gap to ignite the compressed air and fuel mixture in the cylinder.

Your engine needs three ingredients to run: air, fuel, and ignition. Air is funneled toward the cylinder's combustion chamber through the intake system, while the fuel system delivers fuel to either the intake or directly into the chamber, creating a combustible mixture. To ignite that mixture, the ignition system delivers high voltage to the spark plug, which protrudes into the cylinder, and the tip uses electrodes separated by a small gap.

That voltage doesn't come from the battery directly. The ignition coil steps up the car's normal 12-volt supply to a much higher voltage before it ever reaches the plug. The voltage at the spark plug can range anywhere from 20,000 to more than 100,000 volts, which is enough to jump the gap and create a visible spark. High voltage is necessary to jump the gap and produce the spark, and each spark typically lasts about one one-thousandth of a second, repeating thousands of times per minute.

The spark itself converts stored chemical energy in the air and fuel mix into heat, then into mechanical motion that drives your crankshaft and, eventually, your wheels.

Where Are Spark Plugs Located?

Spark plugs are threaded directly into the top of the engine's cylinder head, one plug per cylinder. On most four-cylinder engines, they sit under the ignition coils or plug wires along the top of the engine, usually visible once you remove the plastic engine cover. On V6 and V8 engines, half the plugs sit on each cylinder bank, which can make the rear-facing plugs harder to reach.

Do Diesel Engines Have Spark Plugs?

No, diesel engines do not use spark plugs. Diesel engines rely on compression alone to ignite fuel, though many use glow plugs to assist cold starts.

Diesel engines do not use spark plugs. They operate using very high compression, which creates very high temperatures, to auto-ignite diesel fuel. Instead of a spark, the extreme pressure inside a diesel cylinder heats the air enough that injected fuel ignites on its own.

Some diesel engines do use glow plugs to help create additional heat when starting, especially in cold weather, which matters for Canadian winters. So if you drive a diesel truck or SUV, you can skip the spark plug maintenance schedule entirely, but glow plugs are worth keeping an eye on instead.

Types of Spark Plugs

The four main spark plug types are copper, platinum, double platinum, and iridium, and they differ mainly in electrode material, lifespan, and cost.

• Copper spark plugs: The most affordable option and the softest metal, so they wear fastest. Common in older vehicles and high-performance applications that call for frequent changes.
• Platinum spark plugs: A more durable single-platinum tip that resists wear better than copper. A modern vehicle with platinum spark plugs should last at least 50,000 miles.
• Double platinum spark plugs: Platinum on both the center and side electrode, designed for engines with waste spark ignition systems.
• Iridium spark plugs: The longest-lasting and most heat-resistant option. Iridium spark plugs are good for up to 100,000 miles and sometimes more. They cost more upfront but need replacing far less often.

How to Choose the Right Spark Plug

Always start with your owner's manual or a fitment lookup tool, since spark plug type, heat range, and thread size are specific to your engine and shouldn't be guessed.

Choosing the right spark plug comes down to three things: material, heat range, and fit.

• Match the material to your driving habits. If you keep your car a long time and want the fewest replacements, iridium or double platinum is worth the extra cost. If you're on a tighter budget or drive an older vehicle already spec'd for copper, there's no need to upgrade.
• Respect the heat range. Spark plug heat range is defined by the plug's ability to dissipate heat from the combustion chamber, and the firing end must stay hot enough to prevent fouling but cool enough to prevent pre-ignition. A plug with the wrong heat range for your engine can cause misfires or premature wear even if everything else about it is correct.
• Confirm thread size and reach. Spark plugs aren't universal. The thread diameter, reach into the cylinder, and seat type (gasket or tapered) all need to match your engine exactly, which is why checking your owner's manual or a proper fitment guide before buying is non-negotiable.

5 Signs of a Bad Spark Plug

The most common signs of a bad spark plug are engine misfires, rough idling, poor fuel economy, difficulty starting, and a lit check engine light.

1. Engine misfires. If your car feels like it's stuttering or shaking at a red light, a bad plug is the primary suspect, since a misfire happens when the plug fails to ignite the mixture and the cylinder temporarily loses power.
2. Rough idling. Shaking or bouncing while your engine runs at idle can be caused by bad spark plugs, since failing plugs burn fuel at an inconsistent rate, which is especially noticeable at a stop.
3. Poor fuel economy. Deteriorated spark plugs can account for as much as a 30 percent loss in fuel economy, so a sudden drop in mileage is worth checking against your plugs before anything else.
4. Difficulty starting. Fouled spark plug electrodes may have trouble igniting the fuel mixture, which can show up as an engine that cranks fine but struggles to actually start, especially on cold mornings.
5. Check engine light. Worn or failed spark plugs can trigger the check engine light, and a flashing light specifically can indicate misfires severe enough to risk catalytic converter damage.

Can a Car Start Without Spark Plugs?

No. A gasoline engine has no way to ignite the air and fuel mixture without a spark, so a car with missing or completely dead spark plugs will not start at all.

Can I Drive With a Bad Spark Plug?

Technically yes for a short distance, but it's not recommended. A misfiring plug causes rough performance, wastes fuel, and can send unburned fuel into your catalytic converter, risking expensive damage the longer it continues.

How to Test a Spark Plug

The most reliable way to test a spark plug at home is with a multimeter set to measure resistance across the plug's electrodes, alongside a visual inspection for cracks, wear, or fouling.

To test with a multimeter: let the engine cool, clean around the plug, then use a spark plug socket and ratchet to remove it. Set your multimeter to the lowest ohm range, then place one probe on the center electrode and the other on the metal shell.

A healthy plug typically shows a reading between 4,000 and 8,000 ohms, though your owner's manual may list a manufacturer-specific range. A reading far outside that range, at zero, or showing infinite resistance (often displayed as "OL") points to a plug that needs replacing.

Pair the resistance test with a visual check. Look for a bright blue or white spark across the gap if you're testing while the engine runs; a strong spark indicates the plug is functional, while a weak or absent one suggests it needs replacing. Also inspect for cracks, heavy carbon buildup, or an electrode gap that's visibly worn or widened.

How to Check a Spark Plug Coil

Since ignition coils and spark plugs work as one circuit, a coil problem can mimic a bad plug. Faulty spark plug wires or a weak coil can mimic bad spark plug symptoms, including rough idle, hesitation, misfires, and poor fuel economy, so it's worth ruling the coil out before assuming the plug itself is the problem.

A basic coil check involves measuring primary and secondary resistance with a multimeter and comparing the readings to the manufacturer's spec, or swapping a suspect coil with a known-good one from another cylinder to see if the misfire follows the coil.

How to Clean a Spark Plug

If a plug shows light carbon fouling but the electrode and gap are otherwise intact, cleaning can buy some extra life:

1. Remove the plug with a spark plug socket, keeping track of cylinder order.
2. Use a wire brush or a dedicated spark plug cleaning tool to gently scrub away carbon deposits from the electrode and insulator tip.
3. Wipe away loose debris with a clean cloth or compressed air; avoid sandblasting-style cleaners that can leave abrasive residue in the cylinder.
4. Recheck the electrode gap before reinstalling, since cleaning can shift it slightly.

Cleaning is a short-term fix at best. If the electrode is worn, pitted, or the plug is oil-fouled from a mechanical issue elsewhere in the engine, replacement is the better call.

How to Gap a Spark Plug

Gapping a spark plug means adjusting the space between the center and ground electrode to your engine's exact specification, usually done with a gap gauge before installing a new plug.

Even new spark plugs often ship with a gap that needs adjusting for your specific engine.

1. Check your owner's manual or under-hood sticker for the correct gap specification, usually listed in millimeters or thousandths of an inch.
2. Use a wire-style or coin-style gap gauge to measure the current gap between the electrodes.
3. If the gap is too wide, use the gauge's bending tool to gently press the ground electrode closer to the center electrode.
4. If the gap is too narrow, use the gauge to carefully pry the ground electrode outward.
5. Recheck the gap after each adjustment until it matches spec exactly, since even a small deviation can cause misfires or a weak spark.

What Size Is a Spark Plug Socket?

Most passenger vehicles use either a 5/8 inch (16mm) or 13/16 inch (21mm) spark plug socket, though some engines use less common sizes. Spark plug sockets are deep and rubber-lined to protect the ceramic insulator, which is different from a standard deep socket. Always confirm the exact size for your engine before buying, since using the wrong socket risks cracking the plug's insulator on removal.

How Often Should Spark Plugs Be Changed?

Most spark plugs should be replaced between 30,000 and 100,000 miles, depending on the plug material and your vehicle manufacturer's recommendation.

NAPA's professionals recommend replacing spark plugs every 30,000 miles for many vehicles, which lines up with older copper and standard platinum plug intervals. On the higher end, extended-life spark plugs installed at the factory are usually rated for a lifespan of around 100,000 miles, which is typical for iridium and double platinum plugs in modern engines.

Since intervals vary significantly by plug type and vehicle, always confirm the manufacturer's recommended interval in your owner's manual rather than relying on a single blanket number.

How to Replace a Spark Plug (Step-by-Step)

Replacing a spark plug involves removing the ignition coil or wire, unscrewing the old plug with a spark plug socket, gapping the new plug, and torquing it back in by hand before finishing with a torque wrench.

1. Let the engine cool completely, then locate the spark plugs, typically under the ignition coils along the top of the engine.
2. Pull off the plug boot by holding the boot itself, not the wire, twisting gently before pulling straight off.
3. Remove any ignition coil bolts and lift the coil clear of the plug well.
4. Use a spark plug socket and extension to unscrew the old plug, turning counterclockwise.
5. Compare the part number on your new spark plug to the old one to confirm it's an exact match.
6. Check and adjust the gap on the new plug before installing it, as covered above.
7. Thread the new plug in by hand first to avoid cross-threading, then snug it with a torque wrench to the manufacturer's spec.
8. Reinstall the ignition coil and reconnect any electrical connectors, then repeat for each cylinder.

If your vehicle's plugs are buried under intake components on a V6 or V8, or you're not comfortable with torque specs, having a mechanic handle the job is a reasonable call rather than risking a stripped thread in the cylinder head.

How Much Does Spark Plug Replacement Cost?

Spark plug replacement in Canada typically runs between $100 and $300 for parts and labour on most four-cylinder vehicles, with V6 and V8 engines costing more due to added labour time.

Parts alone are inexpensive, often just $10 to $25 per plug depending on the material, with a full four-cylinder set landing well under $100. Labour is where the range widens. Easily accessible four-cylinder engines might only take an hour, while V6 and V8 engines with plugs buried under intake manifolds or in the back of the engine bay can take two to three hours or more.

As always, get a quote from your specific shop, since labour rates and accessibility vary by vehicle and region.

FAQs

• Does changing spark plugs really make a difference?

Yes. Worn spark plugs cause incomplete combustion, which shows up as lost power, poor fuel economy, and rough running. Fresh, properly gapped plugs restore consistent ignition and often noticeably smoother acceleration.

• Does a spark plug improve performance?

A healthy spark plug ensures complete, well-timed combustion, which supports full engine power output. Upgrading to a higher-quality plug (like iridium) mainly improves consistency and longevity rather than adding horsepower on its own.

• Does a spark plug keep an engine running?

Yes, indirectly. The spark plug ignites each combustion cycle, and without that spark, the engine cannot sustain the cycles that keep it running.

• How many spark plugs are in a car?

Almost always one spark plug per cylinder, so a four-cylinder engine has four plugs, a V6 has six, and a V8 has eight.

Conclusion

Whether you're tackling a full ignition tune-up or tackling other routine maintenance at the same time, PartsBlue carries the ignition coils and ignition coil kits you'll need to pair with a fresh set of spark plugs. And since a lot of drivers use a plug replacement as a chance to check other wear items, it's also a good time to inspect parts like suspension ball joints and shock absorbers, which wear out on a similar mileage-based schedule.

A spark plug might be one of the smallest parts under your hood, but it's also one of the most essential. Understanding what it does, watching for the warning signs above, and staying on top of your replacement interval will keep your engine running smoothly for years to come.