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When Should You Replace Spark Plugs? (Explained)

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  • 7 min read

Last Updated on: 7th September 2023, 12:40 am

Does your vehicle ever misfire or have a rough idle? You may need to replace your spark plugs.

Over time, carbon deposits build up on the spark plugs, they become weaker and will eventually stop working.

In this article I’ll explain how often you should replace your cars spark plugs and how to spot worn out spark plugs.

how often to replace spark plugs

How Often to Replace Spark Plugs

There are two main types of spark plugs.

  1. Traditional
  2. Long-life

Long life spark plugs are usually iridium or platinum tipped and last up to 2-5x longer than traditional ones which are made from copper.

In most cases, traditional spark plugs need to be replaced between 30,000 and 50,000 miles. Long-life spark plugs (with iridium or platinum tips) need to be replaced between 60,000 and 150,000 miles.

If you’re unsure of when to replace your spark plugs, refer to the owners manual found inside of the car or online, it will tell you the service interval and spark plug replacement interval.

However, if an issue arises you may need to replace the spark plugs sooner than expected.

new spark plugs

Warning Signs of Worn Spark Plugs

There are a few obvious signs of worn spark plugs, including the following.

  • Rough idle
  • Misfiring
  • Engine won’t start
  • Reduced power and acceleration
  • Lower fuel economy

In certain vehicles, the engine may not start as readily as it used to, it may also run a bit rough or hesitate and judder when accelerating, this is more pronounced if it’s cold outside.

Fuel economy may be reduced by old and worn out spark plugs. However, there are often minimal symptoms until one of the spark plugs fail, causing the engine to misfire.

Misfiring refers to a failed combustion event in the cylinder. Sometimes a misfire can be consistent or it can be on and off, depending on the reason for failure.

A misfiring engine runs rough, rattles, and lacks power, the “check engine” light may also flash up on modern cars.

A worn-out spark plug might sometimes cause an ignition coil to fail. The ignition coil is the component that produces high-voltage impulses causing the spark plug to spark.

These are not the only ways to find a worn out spark plug, you can also visually inspect a one for worn parts, damage, and build up on the plug.

Can You Check the Spark Plugs?

Visually inspecting a spark plug may be the best way to determine a failing plug.

If an engine is displaying signs of worn spark plugs they are usually examined anyway. A technician can learn a lot from the condition of a spark plug.

For example, black greasy deposits on a spark plug indicates the presence of oil in the combustion chamber, this can be a warning sign of other faults with the vehicle.

spark plug wear chart

If one spark plug is worn but the others seem to be fine, the issue is limited to that cylinder.

One sign of clogged spark plugs is when the engine hesitates, splutters, and runs rough at idle or while cold, but clears up after revving or travelling on the highway.

The faulty spark plugs should be changed, but if the issue persists, the engine must be checked for other underlying faults.

If a spark plug begins to burn or melt, it indicates that the cylinder is running hot, either owing to a cooling system issue, an excessively low air-fuel combination, or detonation.

Further Reading: Engine Cylinder Knock and Detonation

An old spark plug in a modern engine should exhibit no overheating damage and should have minimal deposits and build up.

How to Remove, Inspect & Replace a Spark Plug

Removing and checking a spark plug is a simple task, though it may be challenging for a beginner who’s never done it before.

Below is a video showing you how to remove, inspect and replace the spark plug.

YouTube video

If the gap on your spark plug isn’t set to the correct specification, you might get misfires or knock or see carbon build-up or excess fuel.

When installing new spark plugs, always ensure a proper gap.

Are you wondering how much it costs to replace a spark plug before you commit to it?

How Much Does It Cost to Replace Spark Plugs?

Traditional copper spark plugs are the least expensive variant, priced between $6-10 or £4-8 per spark plug.

Platinum or iridium spark plugs are more expensive, around $15-40 or £10-30 per plug. The labour costs are determined by the engine design.

A repair shop may charge anywhere between $60-120 or £40-100 for labour to replace spark plugs in a 4-cylinder engine, which is a very straightforward operation.

Other engine types may be more expensive and complicated due to different layouts and a higher number of spark plugs needed, for example, V6s or inline 6s.

Sometimes, various components need to be removed to gain access to the spark plugs, such as the intake manifolds, induction pipes, air box, etc, this may raise the cost even more.

Some people wonder whether they can change them early as a precaution.

Can I Change Them Early?

Of course you could replace them early, but is it worth it? Probably not.

The difference in power and fuel economy from a normally functioning spark plug but late on in its lifespan compared to a brand new plug is likely less than 1-2%.

Further Reading: Horsepower Loss Over Time

If your car’s long-life spark plugs are due at 100,000 miles and your dealer suggests replacing them at 50,000 miles, it’s too soon and probably not worth it.

Providing there are no issues such as misfiring, unreasonable fuel economy, stuttering, etc, there is no need to replace spark plugs early.

spark plugs being replaced

Can Spark Plugs Last Longer Than Expected?

Iridium and platinum tipped spark plugs have been known to last longer than the recommended service interval.

However, worn spark plugs are more prone to cause ignition coil problems.

Long-life spark plugs have longer lasting electrodes, however they can often fail due to carbon deposits on the electrodes.

Because of that, it is recommended to replace the spark plugs within their recommended service interval and not leave it too late.

Can I Clean Them Instead?

Cleaning spark plugs and adjusting the electrode spacing instead of replacing them was a typical maintenance task in older vehicles.

However, this has since changed. Spark plugs are no longer designed in a way to be repaired or cleaned in modern vehicles and must be replaced with new ones.

Besides, it would be less expensive to have new spark plugs installed than it would to have the old ones cleaned.

cleaning spark plugs

Should I Replace All Spark Plugs at the Same Time?

It is recommended to replace all spark plugs at the same time. New spark plugs can be made slightly differently than old ones, meaning the ECU may have to make adjustments.

If the ECU has to make adjustments and the spark plugs are all slightly different it could cause issues, it would also be more efficient to replace them all at the same time.

Make sure to thoroughly examine new spark plugs before installing them. If a new spark plug is damaged and then installed, it could cause much worse damage to the engine.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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