When Do the Spark Plugs in Your Car 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 signs of spark plug failure are the same or similar to other problems, so it’s important to consult a mechanic.

In a nutshell: Spark plugs can fail due to carbon build-up, repeated overheating, improper plug gapping, or a coolant leak.

How often should you change your spark plugs?

Spark plugs typically last between 30,000 and 150,000 miles. You should change them at the manufacturer's recommended replacement interval, found in your vehicle handbook, even if you're not experiencing any symptoms of spark plug failure.

Below are some examples of recommended replacement intervals. These are a guide only - you should always check your owner's manual for up-to-date information about your specific car.

What makes your spark plugs go bad?

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

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:

• Increased fuel consumption

• Engine surging

• Jerky ride when driving

• Loss of power and acceleration

• Rough idling

• Engine misfiring

• Trouble starting the engine

• Higher emissions

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.

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

Yes, it could. 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 expert mechanics in our network who can diagnose the problem for you.

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• Eight reasons your car won't start

• What to do when your car won't start

• Why does my car lose power?

• How much does it cost to change spark plugs?