Otto Explains — Spark Plugs
What Do Spark Plugs Do, and When Should They Be Replaced?
Short Answer
A spark plug creates the spark that ignites the air and fuel in each cylinder of a petrol engine, using electrical energy from the ignition coil. Plugs wear gradually, and how long they last depends on the engine and the plug type, so the replacement interval comes from your manufacturer’s schedule rather than a universal figure. Worn plugs or a failing ignition coil can cause misfiring, a rough idle, hard starting and higher fuel use, but those signs have other causes too, so the cause is diagnosed before parts are replaced.
How does a spark plug work?
In a petrol engine, compressing the air and fuel heats the mixture, but not enough to ignite it. The spark plug supplies what is missing: a spark that jumps the small gap between its centre and ground electrodes at the right moment in each cylinder. The energy comes from the ignition coil, which turns the battery’s low voltage into the much higher voltage the spark needs. On most modern engines each plug has its own coil, fitted directly on top of it. Because every combustion starts with that spark, its quality affects how easily the engine starts, how smoothly it runs, its performance, its fuel use and its emissions.
What is the difference between copper, platinum and iridium plugs?
Almost every spark plug has a copper core in its centre electrode, because copper conducts electricity well. Copper is too soft for the tip, though, so a standard plug, often called a copper plug, has nickel-alloy electrodes. These wear faster than precious metals and are replaced sooner. Platinum and iridium plugs have tips made of harder, more heat-resistant metals that erode more slowly, so they last longer between replacements; iridium is the harder of the two and has a higher melting point. Their finer tips also make it easier to create a spark. The right plug is not simply the most expensive one: each engine has its own spark plug specification, developed with the car maker, and the plug fitted should match it.
Why do spark plugs wear out?
Every spark removes a tiny amount of metal from the electrodes. Over millions of sparks the gap widens and the sharp edges round off, so the coil has to produce a higher voltage to make the spark jump. That puts more strain on the ignition coil and makes misfires more likely. Age is the usual reason a plug reaches the end of its life, but plugs can also fail early from carbon build-up, overheating, oil reaching the combustion chamber or an incorrect gap. A technician can read the condition of a removed plug for clues, because deposits on it can point to a problem elsewhere, such as a fuel mixture that is too rich.
What are the signs of worn spark plugs or ignition coils?
Worn plugs can cause misfiring, a rough or uneven idle, difficulty starting, a loss of power and higher fuel consumption, and they can bring on a warning light. A failing ignition coil produces much the same picture: misfiring, a rough idle, reduced power and, in some cases, a car that won’t start, usually with the check engine light on. None of these signs points to the plugs alone. Fuel injectors, air leaks, low compression and faulty sensors can cause the same symptoms, so the first checks are whether the plugs are the correct type and within their working life, and which cylinder and system is actually at fault.
What should I do if the check engine light is flashing?
A steady check engine light means a fault has been recorded and should be diagnosed soon. A flashing check engine light is more urgent. It usually means the engine is misfiring badly enough for unburnt fuel to pass into the catalytic converter, where it can ignite and cause expensive damage. If the light is flashing, or the engine is misfiring severely or losing a lot of power, reduce load and stop safely as soon as practicable rather than continuing your journey, then arrange for the car to be assessed. For more on the difference, see what steady and flashing check engine lights mean.
How often should spark plugs be replaced?
There is no single answer. How long a plug lasts depends on its electrode material, the engine and its condition, and whether it was fitted correctly, which is why long-life platinum and iridium plugs have much longer intervals than nickel ones. The interval to follow is the one in your manufacturer’s maintenance schedule or service book for your engine. Many schedules also set a shorter interval for severe use, which can include long periods of idling, slow driving in heavy traffic, frequent stop-and-go driving, hill and mountain roads, towing, and taxi or commercial use. If any of these describe how your car is driven, check whether your owner’s manual asks you to follow the severe schedule. When plugs fall due as part of a larger service is covered in what a major car service includes.
When should a professional look at the spark plugs?
Replacing plugs on schedule is routine maintenance, but fitting them properly matters: the plug must be the right type, seated correctly and tightened correctly, and mistakes can cause poor fuel economy, fouling or engine damage. AutoTronix Express includes spark plugs in its periodic maintenance, inspecting them and replacing them at the correct interval from your vehicle’s manufacturer schedule, and its major service includes them too. If your car is already showing symptoms, such as misfiring, a rough idle or a warning light, diagnosis comes first: AutoTronix Lab traces an engine warning light to its root cause, and AutoTronix Garage confirms an engine fault before recommending any repair. Once the cause is known, AutoTronix replaces ignition coils as well as plugs when they are at fault. For the plugs themselves, AutoTronix usually recommends OEM plugs, or high-performance plugs where the engine suits them, and the final choice is yours.
Common Questions
- Do diesel engines have spark plugs? No. A diesel engine ignites its fuel with the heat of compressed air, so it has no spark plugs. Many diesels have glow plugs, which help ignition, but they are a different part and do not create a spark.
- Can a worn spark plug strain the ignition coil? Yes. As a plug’s gap widens with wear, the coil has to produce a higher voltage to make the spark jump, which puts more stress on it and makes misfires more likely. Replacing plugs on schedule avoids running with an overly wide gap.
- Should I fit iridium plugs if my car came with nickel plugs? Fit the type your manufacturer specifies for your engine. Each engine has its own spark plug specification, so a plug that suits one car is not automatically right for another. If you are considering a different type, check that it is approved for your engine before it is fitted.
- Why do some cars need new plugs sooner than others? Because plug life depends on the electrode material, the engine and how the car is used. Nickel plugs wear faster than platinum or iridium, and manufacturers often shorten the interval for severe use such as heavy traffic and long idling. Early failure can also come from carbon build-up, overheating or oil reaching the plugs.