Spark plugs: when to change them and how to spot wear
In short
Conventional plugs with a copper core and nickel shell are replaced every 20,000–30,000 km, platinum plugs every 40,000–60,000 km, and iridium plugs last 60,000 to 100,000 km, with some approved by the carmaker up to 120,000 km. The interval that actually applies is the one in your owner's manual, and plugs are always replaced as a complete set, never one at a time.
The interval depends on the electrode material
There is no single figure that covers every car, because spark plugs are not all built the same. The difference comes down to the metal at the tip of the centre electrode: the harder and more resistant it is to spark erosion, the longer the plug keeps its shape and its correct gap. Conventional plugs, with a copper core and a nickel shell, are the cheapest and are typically replaced every 20,000–30,000 km. Platinum plugs, where the tip carries a disc of platinum, generally last 40,000–60,000 km, and double platinum versions, with platinum on the ground electrode as well, go further. Iridium plugs have the finest centre electrode — often under 0.6 mm — and last between 60,000 and 100,000 km, with some approved by the carmaker up to 120,000 km. These numbers are a reference point, not a rule. The interval that counts is the one printed in the owner's manual, because it accounts for boost pressure, the ignition system and the type of injection. A turbocharged direct-injection engine can demand new plugs considerably sooner than a naturally aspirated engine fitted with exactly the same plug type.
The signs that plugs have worn out
Plug wear does not announce itself suddenly; it is a slow decline that many drivers blame on bad fuel. The first symptom is hard starting, particularly on a cold morning, when the starter turns the engine over a couple of extra times before it catches. Next comes a rough idle, with vibration felt through the steering wheel and the gear lever at traffic lights, then misfires under load — a brief stumble under hard acceleration or on a climb. Fuel consumption rises by a few percent, because unburnt mixture ends up in the exhaust, and throttle response becomes lazy. When a cylinder misfires consistently, the engine control unit lights the check engine lamp and stores a code from the P0300 family, for random misfires, or P0301–P0304, where the last digit identifies the affected cylinder. This stage should not be ignored: unburnt fuel reaching the catalytic converter overheats it, and a destroyed converter costs far more than a set of plugs. A plug removed from the engine tells its own story. A light brown, dry deposit is normal. Velvety black means a rich mixture or a clogged air filter, wet oily traces point to worn rings or valve stem seals, and a melted electrode or cracked insulator indicates an ignition problem or a plug with the wrong heat range.
The electrode gap: millimetres that matter
Between the centre electrode and the ground electrode there is a calibrated gap, usually between 0.7 and 1.1 mm depending on the engine. The spark has to bridge exactly that distance, and the voltage required rises in direct proportion to it. The problem is that every spark erodes a little metal — tens of millions of times a year. The gap slowly widens, the coil has to produce ever higher voltage, it runs hotter, and eventually it fails. Many burnt-out coils are not manufacturing defects but casualties of plugs left in the engine tens of thousands of kilometres past their interval. Modern iridium and platinum plugs come pre-gapped from the factory and are not adjusted: the fine electrode bends or snaps easily. On nickel plugs the gap can be checked before fitting with a proper feeler gauge, but never with a coin or an improvised tool.
Heat range: why "higher performance" is not better
The heat range describes how quickly a plug transfers heat out of the combustion chamber and into the cylinder head. A "cold" plug sheds heat fast and is designed for powerful engines that run at high temperatures. A "hot" plug retains heat and self-cleans better in engines that are worked gently. This is where the most common mistake appears: the belief that a "colder", racing plug must automatically be better. Fitted to an ordinary engine, it never reaches self-cleaning temperature, coats itself in carbon and causes exactly the misfires you were trying to avoid. Conversely, a plug that is too hot in a turbocharged engine becomes a source of pre-ignition, and detonation breaks pistons. Thread length and the type of seat — tapered or with a washer — matter just as much. A plug with too long a thread can foul a valve, while one that is too short leaves a step of exposed thread in the head that fills with carbon. The code recommended by the manufacturer is a technical specification, not a marketing suggestion.
Why the whole set is replaced, never a single plug
Plugs in one engine wear almost identically, because they run through the same firing cycles. If you replace only the plug in the cylinder that is misfiring, the rest are close to the end of their life and will fail within months — usually at an inconvenient moment. More importantly, a mixed set means different resistances and different gaps from cylinder to cylinder. The engine runs unevenly, and the control unit tries to compensate with fuel trims that make consumption worse. The price difference between one plug and a full set is small; the difference in how the engine runs is not. Two things matter during fitting: the tightening torque from the manual, applied with a torque wrench, and the condition of the coils or leads. An over-tightened plug damages the thread in an aluminium head, while an under-tightened one cannot shed heat and overheats. Anti-seize compound is used only when the plug manufacturer explicitly calls for it — many modern plugs already have a protective coating, and adding paste on top falsifies the real clamping torque.
LPG, short trips and other things that shorten plug life
The interval in the manual assumes average use. In practice, the way people drive in Moldova can shorten plug life noticeably. LPG conversions are the clearest case: gas burns at a higher temperature and erodes the electrodes faster, so the interval is generally cut by roughly a third and plugs suited to gas operation are used. Short urban trips, where the engine never reaches operating temperature, encourage carbon deposits and unburnt oil residue. Dusty roads outside the cities clog the air filter, and a clogged filter enriches the mixture and blackens the plugs. A visual inspection at every second oil change takes a few minutes and gives early warning if the engine has a deeper problem. In our catalogue you can search plugs and filters directly by OEM number, and if you are unsure which type applies, call us with your VIN on +373 22 009 350 and we will check it together.
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Frequently asked questions
Can I fit a new plug only to the cylinder that misfires?
As an emergency fix, yes; as a solution, no. The other plugs have the same wear and will fail soon, and mismatched gaps make the engine run unevenly. A complete set is the only correct option.
Are iridium plugs worth the extra cost?
If the carmaker fitted iridium plugs at the factory, they are mandatory rather than optional. If your car came with nickel plugs, iridium mainly buys a longer interval rather than extra power — worth it if you want to service the engine less often, but only with the correct code for your engine.
How can I tell at home whether a plug has failed?
Remove the plugs and compare them: the one that clearly looks different from the rest points to the problem cylinder. Then read the fault code with an OBD scanner — a P0302, for example, points straight at cylinder two. A rounded electrode, a cracked insulator or oil traces are clear reasons to replace.
Do the coils need replacing along with the plugs?
Not automatically. Coils are replaced only if they are faulty or cracked, but they are worth inspecting at every plug change, because worn plugs overload them. A coil that has run for a long time with an over-gapped plug is a serious candidate for replacement.