Spark plugs are small, cheap, and easy to forget about. Most drivers never think about them until something goes wrong. But when they do go wrong, you'll feel it every time you press the accelerator.
Here's what to watch for and when to act.
What Spark Plugs Actually Do
Each cylinder in your engine has one spark plug. Its job is to ignite the air-fuel mixture at exactly the right moment, thousands of times per minute. When that ignition is weak, late, or missing entirely, you get a misfire. And misfires cause everything from rough idle to serious engine damage over time.
Most modern engines have 4 plugs. Some V6s and V8s have 6 or 8. Performance engines sometimes run two plugs per cylinder.
How Long Do Spark Plugs Last?
It depends entirely on the type:
Copper plugs are the oldest design and the cheapest. They wear out the fastest, typically needing replacement every 30,000 to 50,000 km. Some older vehicles still specify them.
Platinum plugs last longer, around 60,000 to 100,000 km. They're the standard fit on most vehicles sold in the last 20 years.
Iridium plugs are the premium option. Harder metal, finer electrode tip, better spark. Expect 100,000 to 150,000 km from a good set. Many European and Japanese manufacturers specify iridium from the factory now.
The intervals above are guidelines. Hot climates, frequent short trips, and poor fuel quality all shorten plug life. If you're in Queensland doing a lot of city driving, your plugs will likely wear faster than the spec suggests.
6 Signs Your Spark Plugs Are Done
- Engine Misfires
A misfire feels like a brief stumble or hesitation, usually noticeable under acceleration or at low speeds. Your engine will sometimes feel like it's "hiccupping." You may also see the check engine light come on with a P030X misfire code, where X is the cylinder number.
Don't ignore misfires. Beyond the rough driving experience, raw fuel passing through to the catalytic converter will destroy it. A $200 spark plug job can turn into a $1,500 converter replacement if you leave it too long.
- Rough Idle
If your engine shakes or feels unsettled at red lights when it used to be smooth, worn plugs are a common cause. You might feel a vibration through the steering wheel or gear shifter. It's more noticeable on 4-cylinder engines because there are fewer cylinders to smooth things out.
- Hard Starting
Old spark plugs struggle to produce a strong spark, especially in cold mornings. If your car cranks longer than it used to before firing up, or occasionally refuses to start on the first attempt, plugs are worth checking. Batteries get blamed for this constantly, but worn plugs are often the real problem.
- Poor Fuel Economy
Inefficient combustion from worn plugs means your engine needs more fuel to do the same work. If you're noticing your tank isn't lasting as long as it used to and nothing else has changed in your driving habits, plugs could be costing you money at the bowser.
- Slow Acceleration
When you put your foot down and the car hesitates before pulling away, or the acceleration feels flat compared to how it used to feel, spark plugs could be to blame. Fuel injectors, coils, and air filters can also cause this, but plugs are usually the first thing to check.
- The Check Engine Light Is On
Spark plug misfires are one of the most common reasons the check engine light comes on. A scan tool will give you the exact code. P0300 is a random misfire across multiple cylinders. P0301, P0302, and so on point to a specific cylinder. The cylinder number in the code tells you exactly which plug to check first.
Can You Check Them Yourself?
On some cars, yes. On others, getting to the spark plugs requires removing the intake manifold, coil packs, or other components. It varies a lot by make and model.
If you can access them easily, a visual inspection tells you a lot. A healthy plug tip is light grey or tan. Black sooty deposits suggest a rich fuel mixture. White or chalky deposits indicate the engine is running too hot. Oil fouling means you've got a bigger problem with worn piston rings or valve seals.
Gapping is important too. Each plug has a specified gap between the centre and ground electrode. A worn plug will have a wider gap than spec, which weakens the spark. You can check with a feeler gauge, but at the mileage intervals above, replacement is usually better value than trying to revive old plugs.
What Does Replacement Cost in Australia?
Spark plugs themselves are inexpensive. A set of four copper or platinum plugs typically runs $30 to $80 at Repco or Supercheap. Iridium plugs cost more, anywhere from $80 to $200 for a set of four depending on the brand and application.
Labour is where costs vary. On an accessible 4-cylinder, a workshop might charge 0.5 to 1 hour of labour, so $80 to $180 total for parts and labour. On a V6 or V8 where rear bank plugs are awkward to reach, labour can be 2 to 3 hours, pushing the total to $300 to $600.
Some European vehicles (older BMWs and Mercedes-Benz models in particular) are notorious for plugs that are buried under other components. Always ask for a quote before authorising the work.
Which Plugs Should You Use?
Always match or exceed the factory specification. If the manufacturer specifies iridium plugs, fit iridium. Using cheaper copper plugs in a vehicle designed for iridium will shorten the service interval and may affect performance.
Good brands available in Australia include NGK, Bosch, Denso, and Champion. NGK and Denso supply a large portion of the original equipment market in Japan, so they're a safe choice for Toyota, Honda, Mazda, and Subaru vehicles. Bosch works well for European applications.
Avoid generic unbranded plugs. The savings aren't worth the risk of early failure.
Don't Forget the Ignition Coils
While the plugs are out, it's worth checking the ignition coils. Each coil sits on top of a spark plug and generates the high voltage for the spark. Coils fail more often on high-mileage engines, and a failing coil produces exactly the same misfire symptoms as a worn plug.
If you've replaced the plugs and still have a misfire on one cylinder, the coil on that cylinder is the next thing to test.
Ask TorqueBot About Your Car
TorqueBot knows the spark plug type, gap specification, and service interval for your exact make, model, and engine. If you're not sure what your car takes or when it was last done, ask TorqueBot and get a specific answer rather than a guess.
It also pulls up the OBD codes if you're seeing a check engine light, so you know which cylinder to look at first.