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Spark Plug Essentials: A Comprehensive Guide

By R&D
Published on May 3rd, 2024

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A spark plug electrically ignites a compressed mixture of fuel and air in a gasoline internal combustion engine, causing the mixture to rapidly expand as it burns and convert the stored energy of the fuel into mechanical power.

Spark Plug


Spark plugs are located in the engine’s cylinder head(s). Typically, one spark plug is used for each cylinder. Certain vehicles, including late-model Chrysler automobiles and light trucks equipped with “hemi” V-8 engines (5.7, 6.1, and 6.8 liter) and some Mercedes-Benz automobiles with 5.0 liter V-8 engines, employ two spark plugs per cylinder.


A spark plug is comprised of a steel center shaft running from the top to the bottom of the spark plug. At the top of the shaft is the terminal. At the bottom is the center electrode. Except for these two ends, the center shaft is surrounded by a ceramic insulator.

The lower third of the insulator is sealed into the housing, which is a metal base threaded to fit into the cylinder head. The housing has a hex bolt shape above the threads that allows a wrench to turn the spark plug to tighten or remove it. A ground electrode attached to the bottom of the base is shaped like an “L” that extends under the center electrode, separated from it by an air gap.

High voltage released by the ignition coil is routed though the spark plug wire to the spark plug terminal and travels down the center shaft to the center electrode. Initially, the air gap between the center and side electrodes prevents this electrical energy from traveling further.

However, as the voltage released from the coil builds up within the spark plug – to between 40,000 and 100,000 volts - it overcomes the electrical resistance created by the gap and arcs from the center electrode to the side electrode, creating a spark to ignite the air/fuel mixture vaporized in the engine’s cylinder. Heat generated by this combustion is partially absorbed by the spark plug housing and transferred into the cylinder head, to be dissipated by the circulating engine coolant.

Drivability Symptoms

MIL illuminating

The most obvious symptom of spark plug malfunction is misfiring, resulting in sudden loss of power, engine vibration, and illuminating the instrument panel Malfunction Indicator Light (MIL). If misfiring is severe, the MIL will flash on and off, indicating the condition could result in damage to the catalytic converter. Spark plugs that are fouled or worn may also cause hard starting, deteriorated engine performance, and decreased fuel economy.

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Inspection, Test, and Diagnosis

Deposits on a spark plug’s ceramic insulator tip and the center and side electrodes will display evidence of any malfunction affecting spark plug performance. A properly functioning spark plug has thin grey or light brown deposits on the insulator surrounding the center electrode.

A worn spark plug has rounded off electrodes, increasing the width of the gap. Black carbon deposits indicate incomplete combustion caused by either a weak spark or an overly rich air/fuel mixture. If isolated to one or a few cylinders, carbon deposits may indicate defective spark plug wires, decreased ignition coil output for that cylinder, shorted or grounded spark plugs, or leaking fuel injectors. If exhibited on all cylinders, carbon deposits indicate an excessively rich air/fuel mixture that, in turn, may be caused by a defective oxygen sensor or coolant temperature sensor.

Oily deposits are usually caused by worn valve stem seals, worn valve guides, or worn piston rings. If the spark plug tip is wet with gasoline, the plug is not firing. The usual causes are a fouled or defective spark plug, a loose or defective spark plug wire, or defective ignition coil or ignition control module.

Spark plugs with worn electrodes.

The spark plug insulator should be inspected for cracks in the insulator tip and “carbon tracking” lines on the ceramic exterior above the housing. Cracks in the tip indicate an excessively lean air/fuel mixture that ignites early during the compression stroke, causing severe pressure on the plug tip.

The most common cause is an inoperative Exhaust Gas Recirculation (EGR) valve, part of a system designed to add exhaust gases to the combustion chamber to cool it and prevent pre-ignition. Carbon tracks indicate that voltage is travelling over the exterior of the spark plug, rather than through the core, shorting circuiting the plug.

All vehicles and light trucks manufactured for model year 1996 and later employ second generation On-Board Diagnostics (OBD2). On these vehicles, most malfunctions affecting spark plug performance set generic Diagnostic Trouble Codes (DTCs) that can be retrieved with a Code Reader or Scan Tool.

For example, a sticking EGR valve sets generic DTC P0401. These generic codes have the same meaning for all vehicles, regardless of manufacturer. Many manufacturers also specify “enhanced” codes applicable only to that manufacturer’s vehicles to further refine the diagnosis.

The most common DTCs caused by defective or worn spark plugs are misfire codes P0300 for a random misfire (one that cannot be isolated to a specific cylinder) and P0301 through P0312 for misfires in cylinders one through twelve, respectively. Like most generic DTCs, misfire codes isolate a malfunction, not its cause, and are a starting point for diagnosis rather than a guide to what part to replace. Defective or worn spark plugs are only one of several possible causes of these codes.


Replacement of spark plugs does not require reprogramming.


Worn, damaged, or fouled spark plugs should be replaced. Spark plugs are categorized by the type of metal used for the tip of the center electrode: copper, platinum, or iridium.

Copper spark plugs are the least expensive, but have the shortest life span. Platinum is harder than copper, so platinum spark plugs can last up to 50,000 miles in normal use. A double platinum plug adds a pad of platinum onto the steel side electrode directly under the center electrode to increase durability. Iridium is six times stronger than platinum, so iridium spark plugs resist electrode erosion better than platinum plugs.

Spark plug replacement

Each spark plug manufacturer cross-references its products to original equipment spark plug part numbers specified in vehicle owner’s manuals. Replacement spark plugs may be of better quality – for example, iridium instead of platinum. But, to assure that they have proper heat transfer characteristics, replacement spark plugs must cross-reference to the originals.

There are also specialized spark plugs made for specific vehicles. In the 1990’s, Toyota specified a spark plug with dual side electrodes to increase spark plug life span. Replacement with iridium plugs with a single side electrode accomplishes the same goal.

On some Ford light truck models, the original equipment spark plug is a ‘two-piece design,’ in which a metal shroud covers the insulator tip protruding from the housing. In service, these plugs frequently separated from the shroud during removal, leaving the shroud in the cylinder head. Spark plug manufacturers have since introduced a ‘single piece design’ for these Ford models to eliminate this problem.

Accessing spark plugs on modern vehicles can be difficult. Usually, it is necessary to remove the engine’s appearance cover and remove or reposition the air box, alternator, air injection piping, or other components.

On some vehicles, particularly vans with transverse V-6 engines, it may be easier to remove certain spark plugs from underneath the vehicle. If the vehicle is equipped with coil on plug (COP) ignition, the coil or coil pack rests directly on top of the spark plugs and must be removed to access the plugs. On these vehicles, a rubber boot must then be removed from the well into which the spark plug is recessed.

Spark plugs should be removed only when the engine is cool. Removing spark plugs when the engine is hot substantially increases the risk of stripping the threads in an aluminum cylinder head. To remove spark plugs, first disconnect the spark plug wires by simultaneously pulling and twisting the spark plug boot attached to the plug terminal. (Do not pull on the spark plug wire itself, which can damage the wire internally.)

Spark plugs are removed using a socket wrench and “spark plug socket” - a special deep socket with a foam insert that retains the spark plug in the socket during removal and installation. Typically, it is easier to apply the force required to loosen spark plugs with a “breaker bar” socket handle – a long non-ratcheting handle with a swivel head.

After one or two loosening turns, blow compressed air into the area around the spark plug base to clean out any dirt that might otherwise fall into the engine when the spark plug is removed. (Use canned compressed air sold at any office supply store for cleaning computer keyboards if you don’t have an air compressor.) Swivel joints can be used with socket wrench extensions to remove and install spark plugs that are difficult to reach.

Misfiring car spark plug replacement. Repairing of vehicle.

Even though iridium and most platinum spark plugs are pre-gapped at the factory, it is still advisable to use a “gap gauge” available at any auto parts store to verify that the “gap” between the center and ground electrodes of the new plugs matches vehicle manufacturer’s specifications.

The gauge should fit into the gap without applying pressure. (Do not wedge the gauge into the gap, which can damage the electrodes.) If the gap needs adjustment, use the tool included with the gauge to gently bend the ground electrode to the proper gap.

Some replacement spark plugs are furnished with a crushable washer that fits around the threads of the housing. If so, be sure that the old washer has not remained in the cylinder head before installing the new spark plug.

Many modern vehicles employ aluminum cylinder heads. In these vehicles, it is particularly important to tighten spark plugs only to the vehicle manufacturer’s torque specifications using a torque wrench. Over-tightening spark plugs in an aluminum cylinder head can result in seized plugs, which will strip the threads from the head when the plugs are later removed.

Because anti-seize compound is a lubricant that distorts torque readings, many vehicle manufacturers recommend against its use on spark plugs. If it is used, final torque values must be reduced by 30%.

To prevent spark plug boots from sticking to the new spark plugs, apply a light coating of dielectric grease to the inside of the spark plug boots where the boot rests on the ceramic spark plug insulator. Because it does not conduct electricity, dielectric grease should not be applied to the electrical contacts.

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