Everything you need to know about spark plugs

If your car has been misfiring, struggling to start, or using more fuel than usual, the culprit could be something small: your spark plugs. These compact components play an important role in how your engine runs, yet they're often overlooked until something goes wrong.

Diagnosing bad spark plugs is straightforward, and replacing them is one of the least expensive engine maintenance tasks. This guide explains what spark plugs do, how to spot the warning signs of failure, and when to tackle the job yourself versus booking a professional service. Whether you drive a city hatchback or a family SUV, understanding your spark plugs can save you money and keep your engine running smoothly.

What spark plugs actually do

A spark plug is a small, threaded device that screws into the top of each engine cylinder. Its job is simple: it produces a precisely timed electrical spark that ignites the compressed air-and-fuel mixture inside the cylinder, creating the controlled explosion that powers your engine. Spark plugs also transfer heat away from the combustion chamber into the engine's cooling system.

Each cylinder in your engine has its own dedicated spark plug. So if you drive a four-cylinder car, you have four spark plugs; a six-cylinder engine has six, and so on. When even one plug underperforms, the entire engine feels it, through rougher idling, reduced power, or worse fuel economy.

It's worth noting that spark plugs are specific to petrol engines. Diesels use glow plugs instead, which preheat the combustion chamber rather than producing a spark. The diagnostics in this guide apply to petrol vehicles.

How the ignition cycle works

Understanding the ignition cycle helps you appreciate why a worn spark plug causes so many problems. Here's a simplified breakdown of what happens inside each cylinder thousands of times per minute:

• Intake stroke, the piston moves down, drawing in a precise mixture of air and fuel.
• Compression stroke, the piston moves back up, compressing the mixture into a much smaller space, which makes it more combustible.
• Power stroke, at exactly the right moment the spark plug fires. The electrical arc jumps across a small gap between the plug's centre electrode and ground electrode, igniting the compressed mixture. The resulting explosion forces the piston down.
• Exhaust stroke, the piston rises again, pushing spent gases out through the exhaust valve.

Throughout this process, the spark plug tip is exposed to extreme temperatures, often exceeding 2,500°C at the point of combustion. The plug must conduct enough heat away to avoid pre-ignition, while staying hot enough to burn off carbon deposits. This thermal balancing act is central to plug design and is why choosing the correct plug for your engine matters.

Types of spark plugs and thermal performance

Not all spark plugs are the same. The type fitted to your car affects how long it lasts, how efficiently it fires, and how well it handles heat.

Common spark plug materials

Modern iridium and platinum plugs have longer service lives than traditional copper-nickel plugs. Many newer vehicles leave the factory fitted with iridium plugs because they can go 100,000 miles or more before needing replacement. This is an improvement over the general guidance of around 30,000 miles that applies to standard copper plugs.

Hot plugs vs cold plugs

Spark plugs are also rated by their heat range, which describes how quickly they dissipate heat from the tip.

• Hot plugs have a longer insulator nose, which retains more heat. They're designed for engines that run at lower temperatures or under lighter loads, helping the plug stay hot enough to self-clean and prevent carbon fouling.
• Cold plugs have a shorter insulator nose and transfer heat away more rapidly. They're suited to high-performance or turbocharged engines that generate more combustion heat.

Fitting the wrong heat range can cause problems. A plug that runs too hot risks pre-ignition and potential engine damage; one that runs too cold will foul up with carbon deposits. Match the heat range specified in your owner's manual or by the vehicle manufacturer.

Symptom checklist

Here are the five most common signs that your spark plugs need attention:

• Engine misfire, you feel a brief, noticeable stumble or judder while driving, especially under acceleration. A misfire occurs when the spark plug fails to ignite the fuel mixture properly, leaving one cylinder momentarily inactive.
• Knocking or pinging, a metallic rattling sound from the engine, particularly under load like climbing a hill. This can indicate that the air-fuel mixture is igniting at the wrong time, sometimes caused by a worn or incorrectly gapped plug.
• Poor fuel economy, when plugs aren't firing efficiently, unburnt fuel passes through the engine. If you've noticed fuel costs rising without a change in driving habits, worn plugs could be the reason.
• Hard starting, if your car cranks longer than usual before firing up, especially on cold mornings, weak or fouled spark plugs may not be producing a strong enough spark to ignite the mixture.
• Loss of acceleration, the car feels sluggish or unresponsive when you press the accelerator. This is often the combined effect of incomplete combustion across multiple cylinders.

These symptoms of failing plugs can also overlap with other engine issues — a faulty ignition coil, a clogged fuel injector, or a vacuum leak can produce similar effects. That's why diagnosis matters.

Simple at-home checks

You don't need specialist equipment to perform a basic spark plug inspection. Here's what you can do in your driveway:

• Let the engine cool completely. Spark plugs sit in the cylinder head, which gets extremely hot. Wait at least an hour after driving.
• Locate the spark plugs. They're connected to thick ignition leads or individual coil packs on top of the engine. Your owner's manual will show you exactly where they are.
• Remove one plug at a time. Use a spark plug socket (a deep socket with a rubber insert to grip the plug) and a ratchet. Turn anticlockwise to loosen.
• Inspect the electrode and tip. A healthy spark plug should have a light tan or grey deposit on the firing end. Watch for these warning signs:

Plug appearance	What it means
Light tan/grey deposits	Normal — plug is healthy
Black, sooty coating	Carbon fouling — rich fuel mixture or short journeys
Oily, wet deposits	Oil fouling — possible worn valve seals or piston rings
White, blistered electrode	Overheating — wrong heat range or lean mixture
Worn or rounded electrode	End of life — gap has widened beyond specification
Cracked porcelain insulator	Damaged — replace immediately

• Check the electrode gap. Use a feeler gauge or spark plug gap tool to measure the distance between the centre and ground electrodes. Compare it to the specification in your owner's manual. As plugs age, the electrode gap widens, weakening the spark.

Maintenance and replacement guide

Can you clean spark plugs instead of replacing them?

In some cases, yes. Light carbon fouling — the black, sooty deposits that build up from short trips or a slightly rich fuel mixture — can sometimes be cleaned to restore performance. Use a wire brush or specialist spark plug cleaner, and ensure the electrode gap is reset to the correct specification afterwards.

However, cleaning is a temporary measure, not a long-term fix. If the electrodes are visibly worn, rounded, or the gap has widened significantly, cleaning won't restore the plug's ability to produce a strong, consistent spark. In most cases, replacement is the better investment.

When should you replace spark plugs?

Follow your owner's manual; replacement intervals vary depending on the plug material and your specific engine.

As a general rule of thumb:

• Copper/nickel plugs: every 20,000–30,000 miles
• Platinum plugs: every 60,000–100,000 miles
• Iridium plugs: every 80,000–120,000 miles

Your manufacturer's recommended interval should always take priority over generic guidance. Driving conditions matter too — frequent short journeys, stop-start city driving, and towing can all accelerate plug wear.

Parts-buying checklist

Before purchasing replacement spark plugs, confirm the following details (all found in your owner's manual or via your vehicle's registration lookup on parts supplier websites):

• Correct plug type and part number for your specific engine variant
• Thread diameter and reach (e.g., 14mm × 19mm)
• Heat range as specified by the manufacturer
• Electrode gap — some plugs come pre-gapped, but always verify
• Torque specification for installation — overtightening can damage the cylinder head; undertightening can cause blowby or vibration loosening

Consider sourcing plugs from reputable manufacturers to ensure parts meet OEM specifications.

DIY replacement: safety and step-by-step

Replacing spark plugs is within the ability of most car owners, provided you take a few precautions.

Safety checklist before you start:

• Engine is completely cool
• Ignition is off and key removed
• You're working in a well-ventilated area
• You have the correct spark plug socket, torque wrench, extension bar, and anti-seize compound (if specified)
• You've noted the firing order or are replacing plugs one at a time to avoid mixing up leads

Basic steps:

• Disconnect the ignition lead or coil pack from the first plug.
• Use compressed air or a brush to clear debris from around the plug well, you don't want dirt falling into the cylinder.
• Remove the old plug with your spark plug socket.
• Inspect the old plug (use the appearance table above to check for any underlying issues).
• Check the gap on the new plug with a feeler gauge. Adjust if necessary.
• Thread the new plug in by hand first to avoid cross-threading, then tighten to the manufacturer's torque specification.
• Reconnect the ignition lead or coil pack.
• Repeat for each cylinder.

Cost and time estimate

For most four-cylinder cars, a set of quality spark plugs costs between £15 and £60, depending on the material. The job typically takes 30 to 60 minutes for a straightforward engine layout. Some engines, particularly those with plugs buried beneath intake manifolds or engine covers, are more involved and may justify professional fitting.

A professional spark plug replacement at a garage typically runs between £60 and £150 including parts and labour, though this varies by vehicle and plug type.

Frequently asked questions

1. How many spark plugs does my car have?

   One per cylinder. A three-cylinder engine has three, a four-cylinder has four, a V6 has six, and so on. A small number of high-performance engines use two plugs per cylinder (twin-spark), but this is uncommon in standard road cars.

2. Can bad spark plugs damage my engine?

   Prolonged driving with severely worn or misfiring plugs can damage your catalytic converter, as unburnt fuel enters the exhaust system and overheats the catalyst. It can also wash lubricating oil from cylinder walls in extreme cases. Addressing the problem promptly avoids these costly knock-on effects.

3. Do electric or hybrid cars have spark plugs?

   Fully electric vehicles do not, they have no combustion engine. Plug-in hybrids and mild hybrids with a petrol engine do have spark plugs, and they still need periodic replacement, though intervals may be longer due to reduced engine use.

4. Should I replace all spark plugs at once?

   Yes. Even if only one plug appears worn, replacing the full set ensures consistent combustion across all cylinders and avoids having to repeat the job shortly afterwards.

5. Will new spark plugs improve my fuel economy?

   If your old plugs were worn or fouled, fitting fresh ones can improve fuel efficiency, throttle response, and smooth running. It's one of the most cost-effective maintenance tasks for keeping your engine in good shape.

Book your spark plug check or service

If you'd rather leave it to the professionals, Arnold Clark service centres can inspect, diagnose, and replace your spark plugs as part of a full vehicle service. Technicians will also check your ignition system, engine management codes, and overall engine health. Service centres have online booking and can supply parts that match your vehicle's specification.

Book your service online or browse our full range of servicing and MOT options to keep your car running at its best.

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