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Spark Plug

Delivers the spark that ignites the air-fuel mixture, and doubles as a readable record of what is happening inside the cylinder. A genuine wear item — the gap grows continuously from the day it is installed.

Typical cost: $100–$900
Wear item: Yes
Service life: A true consumable that erodes continuously. Replacement intervals are set by the manufacturer and by plug material, and vary widely between engines — conventional nickel plugs need service far sooner than fine-wire iridium ones. Use the interval in the vehicle's maintenance schedule rather than a general figure. Direct-injection and turbocharged engines frequently specify shorter intervals than the same manufacturer's naturally aspirated engines.

What it does

The spark plug is where combustion starts. It provides a precisely sized gap inside the cylinder for the ignition coil's high voltage to jump, and that arc lights the compressed air-fuel mixture. It is one of very few engine parts designed to be consumed and replaced on schedule, because every spark erodes a little metal from the electrodes and slowly widens the gap. A wider gap demands more voltage, and eventually the coil cannot deliver it under load — that is when misfires start. Worn plugs also destroy ignition coils, so ignoring them is not free. A removed plug is also a diagnostic document: its color, deposits, and wear tell you about fuel mixture, oil consumption, and coolant intrusion in that specific cylinder.

A center electrode runs through a ceramic insulator to a ground electrode, forming a fixed gap exposed to the combustion chamber. The coil raises voltage until the mixture in that gap ionizes and becomes conductive, then current flows across as an arc that releases enough energy to start a flame kernel, which propagates through the chamber. Two design properties matter to a driver. Heat range is how quickly the plug sheds heat into the cylinder head: too cold and deposits build up and foul the plug, too hot and the tip can become an ignition source and cause preignition, which destroys pistons. Heat range is engine-specific and is not interchangeable. Electrode material determines wear rate. Copper-core plugs with nickel alloy tips conduct well but erode fastest; platinum resists erosion better; iridium is harder still and allows a very fine center electrode, which lowers the voltage required to fire. Fine-wire iridium and platinum plugs last dramatically longer for exactly that reason, and using a shorter-life plug in an engine designed for a long-life one shortens the service interval accordingly.

How it fails

Normal erosion is the default path: the gap grows, required firing voltage rises, and misfires appear first under load and later at idle. Carbon fouling coats the insulator with conductive soot from over-rich running or repeated short trips, giving the spark a path to ground instead of across the gap. Oil fouling produces a wet, black, oily plug and points at valve stem seals, worn rings, or a failing turbocharger seal. Coolant intrusion leaves a clean, steam-cleaned white tip or a chalky deposit and is a serious finding — it suggests a head gasket or cracked head. Physical damage happens: an insulator cracked during installation, a tip broken off by detonation, or threads damaged by cross-threading or overtightening. Seized plugs are a genuine repair hazard on some aluminum heads, especially when plugs are left in far past their interval, and can pull threads out of the head on removal. And an incorrect plug — wrong heat range, wrong reach, wrong gap — causes misfires and can cause engine damage even though the plug is brand new.

Symptoms when it is failing

Rough idle and a shudder that smooths out at higher rpm, or vice versa
Misfire codes, often on multiple cylinders when plugs are worn as a set
Hesitation or stumble under acceleration
Hard starting, worse when cold or damp
Fuel economy that has slowly declined over tens of thousands of miles
Ignition coils failing repeatedly on the same engine

These symptoms overlap with several other faults. They are a reason to test this component, not evidence that it has failed.

How to test it before replacing it

Pull them and read them — this part is unusual in that direct inspection is the best test. Do it with the engine cold, or at most barely warm. Aluminum cylinder heads expand around the steel plug threads when hot, and removing plugs from a hot aluminum head is one of the main ways people gall, strip, or pull threads out of the head — the exact failure this entry lists as a real repair hazard. Blow or vacuum the debris out of each plug well before the plug comes loose, so nothing drops into the cylinder. Note which cylinder each plug came from and keep them in order. Compare the plugs to each other: uniform light tan or gray deposits across all cylinders indicate normal operation, and one plug that looks different from the rest is a cylinder-specific problem worth chasing. Black and dry means rich or fouling; black and wet with oil means oil control failure in that cylinder; a suspiciously clean white tip means possible coolant intrusion and warrants a cooling system pressure test and a combustion leak test before anything else. Melted or eroded electrodes with speckled aluminum deposits indicate detonation or preignition, which is an engine-damaging condition to investigate rather than a plug to replace. Measure the gap with a wire-type gauge — not a coin or ramp gauge — and compare it to the specification for the exact engine, because the correct gap is vehicle-specific and guessing it causes misfires. On fine-wire iridium and platinum plugs, which is what most modern engines require, measure only: never pry or bend the ground electrode to adjust one, and never drag the gauge across the fine center electrode, because chipping the precious-metal tip or cracking the insulator nose ruins the plug. Most iridium and platinum plugs come pre-gapped from the factory and should be installed as supplied unless the manufacturer specifically calls for adjustment. Compare measured gap to the new-plug specification to judge how much wear has occurred. If misfires persist with good plugs, divide the problem by swapping the plug and then the coil between cylinders and seeing which the misfire follows, and confirm the cylinder can hold compression before buying more ignition parts. On installation, use a torque wrench to the manufacturer's specification, which varies by thread size, head material, and gasket type — overtightening into an aluminum head strips threads, undertightening causes blow-by and overheating of the plug.

Specification values differ between manufacturers and model years. Where a figure is not genuinely standard across OBD-II vehicles, check it against service information for your specific vehicle rather than a generic number.

Where it sits

Threaded into the cylinder head, one per cylinder, reached after removing the ignition coil or plug wire. Access varies enormously: some four-cylinders are a fifteen-minute job, while the rear bank of a transverse V6 can require removing the intake manifold. Certain engines have well-documented plug removal difficulties that turn this into a much larger job — check for known issues on the specific engine before starting. Plug count is not always one per cylinder; some engines use two.

What replacement costs

Typically $100–$900 at a US independent shop.

Parts and labor at a US independent shop, for a full set. A four-cylinder with easy access sits at the bottom. A V6 or V8 requiring intake removal, or an engine with known seized-plug risk, sits at the top. Iridium and platinum plugs cost more per plug but are usually required, not optional — fitting cheaper plugs into an engine specified for long-life ones is a false saving. If a plug seizes and damages head threads, repair cost rises far above this range.

Ranges are wide because access varies enormously between vehicles — the same part can be a twenty-minute job on one engine and a half-day on another.

Trouble codes involving this component