Spark Plug change & replacement made easy

How does a spark plug work?

A spark plug is a small but vital part of a petrol engine’s ignition system. Its job is to create an electric spark that ignites the air-fuel mixture inside each cylinder. This controlled ignition pushes the piston downward, turning the crankshaft and generating the power that drives your car. Without working spark plugs, the engine can’t start or run efficiently.

Each spark plug has a metal shell that screws into the cylinder head and a central electrode surrounded by a ceramic insulator. When high voltage from the ignition coil is applied, a spark jumps across the electrode gap, igniting the fuel mixture. This process happens thousands of times per minute while you drive.

Over time, spark plugs wear down or become fouled with carbon, oil, or fuel deposits, which weakens the spark. That can lead to engine misfires, rough idling, poor acceleration, or reduced fuel economy. Replacing your spark plugs at the recommended interval keeps your engine smooth, efficient, and reliable.

What does a spark plug replacement job involve?

Here’s how a qualified mechanic typically carries out a spark plug replacement:

• The mechanic starts by scanning the ECU for any fault codes related to misfires or ignition issues to confirm that the spark plugs need replacing.
• For safety, the ignition is switched off and the engine is allowed to cool before any work begins.
• Engine covers, air intake parts, or ignition coils are carefully removed to reach the spark plugs.
• Each spark plug is unscrewed using the correct socket to avoid damaging the threads in the cylinder head.
• The mechanic checks the old plugs for wear, oil, or carbon deposits, which can reveal other engine issues like poor fuel mixture or oil leaks.
• New, manufacturer-specified spark plugs are fitted and tightened to the correct torque setting to ensure a perfect seal and consistent spark.
• Once everything is reassembled, the mechanic starts the engine to confirm smooth idling, proper ignition timing, and no misfire codes.

How do I know when I need a spark plug replacement?

Worn or faulty spark plugs can cause several noticeable performance issues. If you spot any of the following signs, it’s a good idea to book an inspection or spark plug replacement with a qualified mechanic:

• Hard starting: The engine struggles to fire up or needs several attempts to start because the plugs aren’t generating a strong enough spark.
• Stalling after start-up: The car starts but cuts out soon after, often caused by inconsistent sparking or incomplete combustion.
• Engine misfires: Hesitation, jerking, or a loss of power—especially under acceleration—are common indicators that one or more spark plugs have failed.
• Rough idling or vibrations: A shaky engine at idle can point to uneven combustion due to worn spark plugs.
• Check Engine Light: The warning light may illuminate if the ECU detects misfires or poor ignition performance. If you notice these symptoms, have a mechanic check your spark plugs checked and replaced if necessary.

Do diesel cars have spark plugs?

No, diesel engines don’t have spark plugs. Instead, they use glow plugs and a different combustion process altogether.
In a petrol engine, spark plugs create a small electric spark that ignites the air–fuel mixture. Diesel engines, however, work through compression ignition. This means air is compressed to a very high pressure and temperature inside the cylinder, and when diesel fuel is injected, it ignites automatically without needing a spark.

Glow plugs in diesel engines serve a different purpose, they preheat the air in the combustion chamber when the engine is cold, helping it start more easily in low temperatures. Once the engine is running and warm, the glow plugs are no longer needed.
So while both systems are part of the engine’s ignition process, petrol engines rely on spark plugs, whereas diesel engines use glow plugs to aid cold starts.

How to test a spark plug

If your engine is misfiring, idling roughly, or struggling to start, testing the spark plugs can help confirm whether they’re working properly. This involves checking both their condition and spark performance.

Safety first: Because spark plugs operate at very high voltage, testing should only be done by a confident DIYer with the right tools but preferably by a qualified mechanic.

Here’s how a spark plug test is typically carried out:

• Remove the spark plug and look for wear, cracks, oil, or carbon deposits. These can reveal misfires, oil leaks, or air–fuel issues.
• Use a feeler gauge to measure the gap between the centre and ground electrodes. If it’s wider or narrower than the manufacturer’s specification, the plug won’t fire efficiently.
• Reconnect the plug to its ignition lead, ground the threaded part against metal on the engine, and have someone crank the engine while you observe (from a safe distance). A strong, blue spark means the plug is healthy; a weak or no spark may indicate a faulty plug or ignition coil.
• With a multimeter, check resistance between the terminal and electrode. A reading outside the typical range (often 4–8 kΩ) suggests internal failure.

If any plug fails these checks or if you’re not sure it’s best to have your ignition system professionally tested. A mechanic can safely identify whether the fault lies with the spark plug, ignition coil, or wiring, and replace the necessary parts to restore smooth engine performance.

How often should I change my spark plugs?

Most spark plugs should be replaced every 20,000 to 30,000 miles for standard copper types, while platinum and iridium spark plugs can typically last 60,000 to 100,000 miles. Premium double platinum or double iridium plugs may last beyond 100,000 miles under normal driving conditions.

Your actual spark plug replacement interval can vary depending on how and where you drive. Frequent short trips, heavy stop-start traffic, towing, or symptoms such as rough idling, poor fuel economy, or difficulty starting can all mean your spark plugs are wearing out sooner.

To get the most accurate recommendation, always check your owner’s manual or ask a qualified mechanic to inspect your spark plugs during routine servicing.

Can a spark plug be repaired?

While you can clean away carbon or oil deposits in a pinch, using a wire brush or specialised spray, this only addresses surface fouling and won’t restore a worn or damaged electrode.

Due to the gap being between the centre, ground electrode widening over time, and heat cycles eroding materials, spark plugs inevitably lose their ability to fire reliably. For this reason, replacing worn or damaged spark plugs is the only way to guarantee proper ignition, fuel efficiency, and smooth performance.

In short, cleaning can be a temporary fix if you’re stuck, but for long‑term reliability and engine health, spark plugs should be replaced according to your vehicle’s maintenance schedule rather than repaired.

How to clean spark plugs?

If your spark plugs are only lightly fouled with carbon or fuel deposits, they can sometimes be cleaned to restore performance, but only as a temporary fix. Cleaning won’t reverse wear on the electrodes or insulation, and heavily worn plugs should always be replaced. Here are some steps you can take but only if you’re confident with basic maintenance skills:

• Inspect the plug first: Remove the spark plug and check for cracks, oil fouling, or worn electrodes. If any damage is present, replacement is the best option.

• Remove surface deposits: Use a wire brush or a specialist spark plug cleaning tool to gently remove carbon buildup from the tip and threads. Avoid sandpaper, as abrasive grit can damage the plug or enter the engine.

• Clean the electrode gap: Carefully scrape or blow away any remaining debris. Compressed air can help clear the area around the insulator and gap.

• Check the gap: Use a feeler gauge to ensure the gap matches your vehicle manufacturer’s specification. If it’s out of range, gently adjust it with a spark plug gapping tool.

• Reinstall carefully: Thread the plug back into the cylinder head by hand to avoid cross-threading, then tighten with a torque wrench to the correct setting.

Important: Cleaning can only help if the plugs are dirty but otherwise in good condition. If the electrodes are eroded or the plug continues to foul quickly, it’s safer and more effective to have them replaced by a qualified mechanic.

How much does a spark plug replacement cost?

On average, replacing your spark plugs costs around £85, with most jobs falling between £55 and £125. The exact price depends on your vehicle’s make and model and the specific spark plugs recommended by the manufacturer. Factors such as local labour rates and whether you choose a mobile mechanic or a traditional garage can also affect the final cost. Use our free price estimator tool to find out what you could save by having your spark plug replaced by ClickMechanic!

What does a ClickMechanic spark plug replacement quote cover?

Unless otherwise stated, the quote will cover:

• The cost of the spark plugs
• Coming to you
• Fitting of the parts
• Disposal of the old parts (unless you request to keep them)
• VAT (if applicable)
• 12-month parts and labour warranty

Can a mobile mechanic replace the spark plug?

Yes, a mobile mechanic can replace your spark plugs right at your home or workplace. It’s one of the most common mobile services and usually doesn’t require a full workshop setup.

Most modern engines use a coil-on-plug ignition system, allowing easy access to each spark plug using standard tools such as a spark plug socket, ratchet, and torque wrench. A qualified mobile mechanic will arrive with the correct replacement plugs, run a diagnostic scan for any misfire codes, replace each plug in sequence, and tighten them to manufacturer specifications, all done safely at the kerbside.

In super rare cases, where spark plugs are difficult to reach due to intake manifolds or special performance components, a garage visit might be recommended. If that happens and your vehicle is drivable, ClickMechanic offers free collection and delivery so the work can be completed without any hassle.