Spark Plug Replacement: Signs and Costs in the UK

Car Owl

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Summary

• Spark plugs last 20,000–60,000 miles: Iridium and platinum plugs last longer than copper ones.
• Worn plugs cause misfires and poor fuel economy: Replacing them is one of the cheapest performance upgrades.
• Replacement costs £40–£200: Depending on the number of cylinders and plug type.

Spark plugs are small but vital. They ignite the fuel-air mixture in each cylinder. When they wear out, your engine runs poorly, wastes fuel, and can even fail its MOT on emissions.

This guide covers the warning signs, replacement intervals, and costs for UK drivers.

What Spark Plugs Do

Each spark plug fires thousands of times per minute. It creates an electrical spark that ignites the fuel inside the cylinder. A healthy spark plug ensures:

• Complete combustion of fuel.
• Smooth engine running.
• Good fuel economy.
• Clean exhaust emissions.

Diesel engines do not have spark plugs. They use glow plugs instead, which work differently.

Signs You Need New Spark Plugs

Watch for these symptoms:

1. Engine misfires: The engine stutters, hesitates, or judders during acceleration.
2. Rough idle: The engine shakes or vibrates when stationary.
3. Poor fuel economy: Worn plugs mean incomplete combustion and wasted fuel.
4. Difficulty starting: Especially in cold weather. Weak sparks struggle to ignite cold fuel.
5. Check engine light: Misfires trigger the engine management warning light.
6. Increased emissions: Unburnt fuel exits through the exhaust, raising emissions.

Replacement Intervals

Always use the type specified in your owner's manual. Fitting the wrong type can cause running problems.

Iridium plugs cost more upfront but last two to three times longer than copper. Over the life of the car, they often work out cheaper.

How Much Does Replacement Cost?

The total cost depends on how many cylinders your engine has:

• 3-cylinder engine: £40–£80 (parts and labour).
• 4-cylinder engine: £60–£120.
• 6-cylinder engine: £100–£200.

Some engines have the plugs buried under other components. This increases labour time and cost. Subaru boxer engines and some V6 layouts are particularly awkward.

Can You Do It Yourself?

On many cars, spark plug replacement is a straightforward DIY job:

1. Let the engine cool completely.
2. Remove the engine cover if fitted.
3. Disconnect the ignition coil pack from each plug.
4. Use a spark plug socket (a deep socket with a rubber insert) to remove the old plug.
5. Check the gap on the new plug matches the specification.
6. Screw in the new plug by hand first, then tighten with a torque wrench.
7. Reconnect the coil pack.

Over-tightening can crack the plug or strip the threads in the cylinder head. Always use a torque wrench.

Spark Plugs and the MOT

Spark plugs are not directly checked during the MOT. But worn plugs cause high emissions, which are tested. If your car fails on emissions, worn spark plugs could be the reason.

Replacing them before your MOT is a cheap way to improve your chances of passing. Read our MOT guide for more tips.

Glow Plugs in Diesel Cars

Diesel engines do not use spark plugs. Instead, they have glow plugs that heat the combustion chamber to help cold starting:

• Signs of failure: Difficult starting in cold weather, rough idle for the first minute, check engine light.
• Replacement interval: Every 60,000–100,000 miles.
• Cost: £80–£250 for a full set, depending on the car.

Like spark plugs, glow plugs degrade over time. If your diesel car struggles to start on cold mornings, worn glow plugs are a likely cause.

Choosing the Right Spark Plug

Using the correct spark plug matters. Here is what to check:

• Heat range: Must match the manufacturer's specification. Too hot causes pre-ignition. Too cold causes fouling.
• Thread size and reach: The wrong size will not fit or may damage the cylinder head.
• Gap: The electrode gap must be set correctly. Some plugs come pre-gapped. Others need adjusting.
• Brand: NGK, Denso, Bosch, and Champion are all reliable choices. Use what your manufacturer recommends.

Your parts supplier can look up the correct plug using your registration number. Always double-check before fitting.

Common Spark Plug Problems

A mechanic can read the condition of your old spark plugs to diagnose engine issues:

• Black, sooty deposits: The engine is running too rich. Check the air filter and fuel injectors.
• White, blistered electrode: The engine is running too hot. Check the cooling system.
• Oil-fouled: Oil is entering the combustion chamber. This could indicate worn valve seals or piston rings.
• Normal tan or grey colour: The engine is running correctly. Simply replace with new plugs.

Keeping old plugs for inspection is always worthwhile. They tell a story about your engine's health.

Final Thoughts

Spark plugs are cheap and easy to replace. Do not ignore the warning signs. Fresh plugs improve power, fuel economy, and emissions.

Check your owner's manual for the correct replacement interval and plug type. If in doubt, ask your garage during the next service.

Keep on top of all maintenance tasks with our car servicing guide.

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