The humble spark plug is the unsung hero of your engine. Its job is to ignite the fuel oxygen mixture in the cylinder head. This generates an explosive force that pushes the piston down the cylinder and turns the crank.

Worn spark plugs can create havoc with this process, causing your engine to misfire, reducing performance and increasing fuel consumption. So it’s vital that you inspect and change your spark plugs regularly.

The problem is the vastly different driving conditions across Australia, can make a big difference in how long your spark plugs last. So how do you know if your spark plugs need changing?

Fortunately, your spark plugs are such an integral part of your engine that problems are easy to spot if you know what to look for. So in this article, we are going to look at 7 symptoms your spark plugs are past their best.

1. Your car has difficulty starting

Starting problems are often blamed on the battery, but spark plugs are just as likely to be the cause. As we already mentioned, spark plugs are an integral part of your engine's ignition system, so any faults here will show up when you start the engine.

A good way to tell if it's the battery or the plugs is to listen carefully when you turn the ignition. If the battery is flat, the engine will fail to turn over. If the engine attempts to turn over but doesn’t spark into life, it’s more likely to be ignition related.

2. Your engine is rough idling

Does your engine vibrate and sound rough when idling? This is a sign one of the cylinders is misfiring. One of the most common causes of a single misfiring cylinder is a faulty spark plug.

Start by checking the ignition lead connections to each of the spark plugs as a loose connection could be the issue. Remove the ignition leads from each of the plugs and reseat them. If the problem persists, take your car to the garage and get it checked over.

3. You have developed a misfire

A misfiring engine is similar to rough idling, but in this case, the issue occurs when the car is moving. A misfiring engine is a sign of a defective ignition system. This could be a sensor problem or a loose connection.

But defective spark plugs can also cause the engine to misfire, so if you notice your engine running rough or misfiring under acceleration, book your car in for a service and get it checked out as soon as possible.

4. Your fuel consumption has increased

Do you find yourself filling up at the gas station more frequently? This is one of the early signs of defective spark plugs. A spark plug that is not creating a large enough spark will not ignite all the fuel in the combustion chamber.

This results in unburnt fuel being left in the cylinder which is then forced out of the exhaust pipe. This not only wastes fuel but also increases carbon monoxide emissions. So, check your spark plugs first if your fuel consumption goes up.

5. Your car doesn't accelerate like before

When you press the accelerator, does your car not respond as quickly as it used to? This could be because the spark plug is not able to generate enough spark to burn the fuel mixture efficiently.

An inefficient fuel burn will result in power being sapped, something that will become more obvious under heavy acceleration. So if you feel power is down and acceleration is muted, get a mechanic to check things over because this will only get worse over time.

6. You have noticed the engine is noisier than normal

Does your engine sound noisier than normal, or has it started making funny noises? This could be a sign that one or more of your spark plugs is not firing properly. This will alter the ignition timing, causing the engine to vibrate more than usual.

An ignition timing fault will cause the engine to run less efficiently, and increase wear and tear on other components. Replacing spark plugs is relatively cheap in comparison. So always get your car checked by a qualified mechanic if it starts making funny noises.

7. The engine warning light is on

Most cars have an engine warning light these days. This provides a preemptive warning that something is wrong. In most cases, the light is triggered by a fuel mixture that is running too rich or too lean.

So if you notice an unusual icon on the dashboard, often in the form of a spanner or engine, get your engine checked as soon as possible. The longer you leave it, the more damage is being done to your engine and the more costly it will be to repair.

How often should spark plugs be replaced?

Spark plugs should be replaced in line with the manufacturer's service schedule. Most manufacturers recommend spark plugs to be changed every 30,000 to 50,000 km, but this is the maximum duration, spark plugs can and do fail at any time.

Spark plugs cost very little compared to other components. By being proactive about spark plug maintenance, you can prolong the life of your engine, reduce running costs and improve reliability. If you notice any of the above symptoms, get your engine checked by a qualified mechanic as soon as possible.

For a great deal on financing your new purchase, contact Australia's best reviewed finance broker on 1300 Credit (1300 273 348) today or visit

Jordan Ballard

Jordan Ballard

Automotive Content Editor

Jordan is a car finance and automotive industry specialist at Only Cars. With over 20 years of experience with frontline and management roles in sales, finance and other areas, Jordan has an incredible understanding of the automotive industry. As Automotive Content Editor, Jordan loves sharing his passion for cars with the Only Cars audience.