When Do the Spark Plugs in Your Car Need Replacing?

Charlotte Phillips, 4 months ago

5 min read

  • Engine
  • Maintenance
Spark plugs

WhoCanFixMyCar outlines what happens when your spark plugs fail and how you can tell they need replacing.

Spark plugs can fail for various reasons; when they do, there are a few symptoms to keep an eye out for.

It’s worth noting that some of the symptoms of spark plug failure are the same or similar to several other problems, so it’s important to always have the problem checked by a professional.


What makes your spark plugs go bad?

What are the symptoms of spark plug failure?

How often should you change your spark plugs?

My car won't start - could it be my spark plugs?

What makes your spark plugs go bad?

Premature failure of your spark plugs can be pinpointed to a number of different causes.

Mechanic holding an old spark plug.

1. Repeated overheating of the spark plug tip

When the engine timing is off, this can result in pre-ignition occurring and causing excessive heat to build up in the combustion chamber, causing the spark plugs to fail.

2. Carbon build-up

When engine oil leaks into the combustion chamber, the tip of the spark plugs can become dirty, and the sooty carbon that builds up over time can cause them to fail.

Potential causes include a rich air-fuel mixture, idling your vehicle for too long, and a weak ignition. You can spot a carbon build-up on your spark plugs as a dry and black substance.

3. Improper plug gapping

If the small gap between the metal on the tip of the spark plug is not the correct size for the engine, it can cause stress on the tip, making it wear down prematurely.

4. Coolant leak

A coolant leak from the head gasket can result in coolant entering the combustion chamber and sticking to the spark plugs, making them malfunction. Depending on your coolant, the deposit may be grey or ashy

What are the symptoms of spark plug failure?

Some of the tell-tale signs that your spark plugs are failing include:

If you notice any of these symptoms, there could be a problem with your spark plugs. Taking your car to a specialist as soon as possible will help avoid any further damage and ensure that the problem is diagnosed correctly.

How often should you change your spark plugs?

As a general rule, if maintained well your spark plugs could last between 80,000 and 100,000 miles.

An old vs new spark plug.

That said, many auto manufacturers recommend that spark plugs should be serviced every 30,000 miles to ensure they continue working safely and efficiently.

My car won't start - could it be caused by my spark plugs?


Spark plugs are a vital part of your car’s ignition system. They work by igniting the air and fuel mixture inside the engine, leading to the combustion process that produces power. When something goes wrong with your spark plugs, the engine can misfire or not start up at all.

Here are eight reasons why your car won't start.

Think you need a specialist to take a look at your spark plugs? There are hundreds of trained specialists in our network who can diagnose the problem for you.

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