A car that cranks longer than usual, hesitates when pulling away or suddenly uses more fuel may be telling you something simple: its spark plugs are due. If you are wondering when should spark plugs change, the short answer is at the interval specified for your vehicle, or earlier if symptoms point to an ignition problem.
Spark plugs are inexpensive compared with the damage and inconvenience a misfire can cause. They create the spark that ignites the air-fuel mixture in petrol engines. When they wear, your engine has to work harder to start, idle and accelerate properly. Replacing them on time is practical maintenance that can help avoid bigger repair bills.
When should spark plugs be changed?
There is no one kilometre figure that suits every vehicle. Plug life depends on the engine, the plug material, driving conditions and whether the correct plugs were fitted last time. Your owner’s manual or service schedule is always the starting point.
As a general guide, standard copper spark plugs often need changing around every 20,000 to 40,000 km. Platinum plugs commonly last around 80,000 to 100,000 km, while iridium plugs can often run for 100,000 to 160,000 km. Some modern vehicles have longer factory intervals, but that does not mean the plugs should be ignored between services.
A Toyota Corolla with iridium plugs, for example, may have a much longer replacement interval than an older Nissan or Holden running conventional plugs. BMW, Subaru, Suzuki and Fiat engines also have their own requirements. Check the exact engine code and year rather than ordering plugs based on the vehicle badge alone.
If the vehicle has no clear service history, treat the plugs as a maintenance item worth checking sooner rather than later. A previous owner may have fitted cheaper plugs with a shorter lifespan, or the plugs may simply be overdue.
Signs your spark plugs may be worn
A worn plug does not always trigger a warning light straight away. The engine may still run, but it will often lose efficiency before it fails completely. Pay attention if your vehicle develops rough idle, hard starting, sluggish acceleration or poorer fuel economy without another obvious cause.
Misfiring is one of the clearest warning signs. You may feel a shudder through the car under acceleration, particularly when climbing a hill, carrying a load or overtaking. The check engine light may flash or stay on. A flashing light needs prompt attention because an active misfire can send unburnt fuel into the catalytic converter and cause an expensive failure.
Other common signs include:
- Longer cranking before the engine starts
- Uneven idle or engine vibration at traffic lights
- Hesitation when accelerating
- Fuel use increasing noticeably
- Loss of power under load
- A petrol smell from unburnt fuel in some cases
Plug material makes a real difference
Spark plugs are not all interchangeable. Their centre electrode material affects service life, price and suitability for the engine.
Copper plugs are generally the most affordable option and can work well in engines designed for them. Their trade-off is a shorter replacement interval. They are common in older vehicles and may suit owners who service their car regularly.
Platinum plugs have a more durable electrode than conventional copper plugs. They usually cost more upfront but can reduce how often the plugs need replacing. They are often specified for vehicles where plug access takes more labour.
Iridium plugs use a very fine, hard-wearing centre electrode. They offer excellent durability and reliable spark performance, which is why many newer Japanese and European petrol engines use them from the factory. They cost more than basic plugs, but replacing long-life plugs with cheaper short-life alternatives can be false economy.
Use the plug type recommended by the manufacturer. Do not downgrade from iridium to copper just to save on the purchase price unless your vehicle’s specifications allow it. On engines where plugs sit beneath the intake manifold or require significant labour to reach, fitting the correct long-life plug can save money over time.
Why waiting too long costs more
As spark plugs wear, the gap between the electrodes increases. The ignition system then needs more voltage to create a spark. That extra demand can put stress on ignition coils, leads and coil boots.
A weak coil may cope with new plugs but struggle once the plug gap becomes too wide. This is one reason a vehicle can begin misfiring after a long period without a tune-up. Replacing the plugs may solve the issue, but a coil already under strain may also need replacement.
Ignoring a consistent misfire is the costly part. Unburnt fuel can overheat the catalytic converter, and severe misfires can make driving unsafe when the car loses power in traffic. Replacing spark plugs at the proper interval is far cheaper than replacing a converter, several coils or dealing with a breakdown.
Driving conditions can shorten plug life
The published interval assumes normal operation. Real-world use can be tougher, especially for vehicles used mainly on short trips. Frequent cold starts mean the engine spends more time before reaching full operating temperature, which can contribute to deposits on plugs.
Stop-start city driving, heavy towing, repeated high-load driving and an engine that burns oil can also affect plug condition. A car used for short school runs and errands may need inspection sooner than one that regularly covers open-road kilometres.
Oil deposits, heavy black soot or a white, overheated-looking electrode can point to an underlying engine or fuelling issue. In that situation, changing the plugs alone may restore running temporarily, but it will not fix the cause. A mechanic can inspect the removed plugs for clues about what is happening inside the engine.
Choosing the right replacement spark plugs
Correct fitment matters. Spark plugs vary by thread size, reach, seat design, heat range, electrode configuration and factory gap. A plug that physically screws in may still be wrong for the engine.
Always match the application to your vehicle’s make, model, year and engine. If there is any uncertainty, confirm the engine details before ordering. This matters particularly with vehicles that have multiple engine options in the same model range.
Avoid guessing the gap or forcing adjustments on fine-wire platinum and iridium plugs. Many modern plugs are pre-gapped for their intended application, and the delicate centre electrode can be damaged by rough handling. Follow the vehicle manufacturer’s specification and use the correct torque during installation. Over-tightening can damage threads in an aluminium cylinder head, while under-tightening can affect heat transfer and sealing.
If you are replacing plugs because of a misfire, inspect the ignition coils and boots at the same time. Look for cracking, corrosion, oil in the plug wells or damaged connectors. Replacing a failed plug without addressing a leaking rocker cover gasket or weak coil can leave you with the same problem soon after.
A practical service decision
For most petrol vehicles, spark plugs are best changed as planned maintenance rather than after a breakdown. Check the service book, confirm which material your engine uses and factor in how the car is driven. If you have rough running, hard starts or a check engine light, do not wait for the next scheduled service to investigate it.
Buying the correct replacement parts early gives you more control over the cost of the job. JBH Auto Parts supplies practical replacement spark plugs and ignition components for many popular NZ vehicles, helping owners keep routine maintenance affordable.
A fresh set of correctly specified plugs can restore smooth starting and dependable performance, but the best result comes from changing them before worn plugs turn a simple service item into a larger ignition repair.