Advice7 min read7 views
Updated 1 November 2026 · Written by the Wahanaya.lk team
Most petrol cars need new spark plugs every 20,000-100,000 km depending on the plug type, but worn plugs show up sooner as rough idling, poor mileage, or a failed emission test. Check your plug type and don't wait for a complete misfire.
Change your spark plugs on the interval set by your car's manufacturer for the plug type fitted, not on a guess. As a rough guide: basic copper plugs need changing every 20,000-30,000 km, platinum plugs stretch to 60,000-100,000 km, and iridium plugs — now common in most Japanese petrol cars including hybrids — can run 100,000-120,000 km before they need attention. Your service book will state the exact figure Toyota, Honda, Suzuki or Nissan set for your engine, so that's the number to follow, not a number you see on a forum for a different model.
If you don't know what plugs are fitted or when they were last changed, that's worth finding out at your next service rather than waiting for a problem. This matters more if you've just bought a used car — ask the seller or check the service history before you commit. Browsing listings on /vehicles and asking for service records is a normal, reasonable question to put to any seller.
Rough idling, hesitant acceleration, and a noticeable drop in fuel economy are usually the first signs. A worn spark plug can't create a strong, consistent spark, so the engine misfires — you may feel the car stutter or jerk when you press the accelerator, especially pulling away from a junction or climbing a hill. Other signs include the engine struggling to start, especially on a damp morning, and the check engine light coming on. If you notice several of these together, get the plugs checked before they get worse — a misfiring cylinder can eventually affect the catalytic converter, which is a far more expensive repair.
Constant stop-start driving in Colombo, Kandy, or any congested town puts more load on plugs than steady highway running does, because the engine spends more time idling and accelerating from a stop rather than cruising at a constant speed. Short trips — the school run, the office, the corner shop — also don't let the engine reach its full operating temperature long enough to burn off deposits on the plug tip cleanly. If most of your driving is short urban hops, lean toward the shorter end of your plug's recommended interval rather than the longer one, and don't be surprised if a mechanic suggests checking them a little earlier than the book says.
Most Japanese petrol cars sold in Sri Lanka, including hybrids, are designed to run on regular 92 octane petrol, and running the recommended grade won't harm your plugs. Using a lower-than-recommended grade for a high-compression engine can cause knocking, which puts extra stress on the ignition system over time, so stick to what your manual specifies rather than always choosing the cheaper option by default. If you're unsure which grade your model needs, your owner's manual or dealer workshop will know — it's a five-minute question that avoids a bigger bill later.
Yes. Worn, damaged, or the wrong model of spark plug is listed as a reason petrol vehicles fail Sri Lanka's emission test, as of October 2026, according to DriveGreen's published testing guidance. A misfiring cylinder sends unburnt fuel straight into the exhaust, which spikes the hydrocarbon (HC) reading the test measures. If your car is due for its emission test — which you need before renewing your revenue licence — it's worth having the plugs inspected beforehand rather than finding out at the testing centre and having to come back.
The most common mistake is buying plugs by price alone rather than by the type your engine actually needs — fitting copper plugs into an engine designed for iridium, or the wrong heat range, can cause more problems than it solves. Another is ignoring early symptoms like rough idling because the car still "runs fine," only to end up with a damaged catalytic converter or ignition coil from a prolonged misfire. People also sometimes over-tighten or under-tighten plugs when doing it themselves, which can strip the thread in the cylinder head or cause a poor seal — both expensive to fix. If you're not confident with a torque wrench and the correct gap setting, it's safer to pay a mechanic for this one.
Cost depends on your engine's number of cylinders and whether you need copper, platinum, or iridium plugs, plus labour if coils or an intake manifold need removing to reach them. Because import costs and the rupee move, any figure you see quoted can be out of date by the time you read it — get a quote from two or three garages for your exact model rather than relying on an old number. If a major repair comes up and you're weighing whether to fix the car or sell it, checking its market value on /price-check first can help you decide.
If you're not sure what's going on with your engine, it's worth posting your symptoms — model, mileage, and what you're noticing — on the /forum and getting opinions from other owners of the same model before you spend money. And if you're generally keeping your car roadworthy for inspections and resale, our /safety-tips page covers the wider checks worth doing alongside this one.
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